: Is it ok to reference something modern to give the reader a better idea of what something looks like if the book is set in the Middle Ages? This is a random example but would it be bad

Normally, non-POV narrators should be invisible. That's generally good because drawing attention to the narrator will tend to pull readers out of the story and remind them that it's only a story.

The times I've seen a visible non-POV narrator work, the narrator spoke playfully or sarcastically.

But you have to remind yourself that you are deliberately distracting the reader from the story when you use this technique, and you have to decide whether that is the effect you want.

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You can do it if you're writing a comedy (ha-ha funny, not phew-happy-ending); in fact, stylistically, it is likely easy to pull off if you are not very full of yourself and you don't want your audience to take you too seriously.

You haven't indicated that you are after that, but I would argue it is at times entirely appropriate.

You can make a lot of interesting comparisons if you allow yourself this space, but you'd want to stay tonally similar and drop these types things early.

The dragon breathed fire the way a teenage girl chewed and popped bubble gum; the knights, in turn, died in the way that most teen romances do at the feet of those who know not yet what they want. Even in the absence of malice the deaths were inevitable messy accidents in the circle of life. Young Roland watched from a hilltop as he did every friday night, hungry for popcorn, though he knew not yet what that was. "Someone pass the mead!" he called boisterously.

Really, it seems like it could be a lot of fun, but probably not the story you're writing.

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The only way this is permissible (in my view, of course) is if the main character or the narrator is actually a time traveler from the future.

It isn't the setting you need to stay faithful to, it is the mind of the narrator, which is usually of the same mind as the Main Character. If your main character came from 2020, sure, they might think of the sword as being as black and shiny as a new car, and the narrator typically knows everything the main character knows. (In 1st person for sure, also in 3rd person omni or limited).

Other than that, this is likely to be so jarring an editor / publisher will reject the work as too amateurish; they likely will not finish reading the sample. (They get far too many submissions to give anybody the benefit of the doubt; one jarring error and off you go to the reject pile).

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If you do this, it will have a very specific effect --it will create distance between the narrator and the setting, which will tend to remind the reader of the artificiality of the writing. You may want to do this, particularly if you are writing postmodernist fiction or meta-fiction, where you want to deliberately call attention to the writer. Another usage is if you want the readers to experience this work primarily as something modern --if you don't want them to be absorbed into the historical setting.

Some writers have used anachronism very effectively --T.H. White's The Once and Future King is probably one of the best examples. The Princess Bride and The Last Unicorn also use the technique (apparently it's a bit easier to pull off in a fantasy narrative, which is intrinsically located in a kind of timeless, mythopoeic realm). Others have gotten away with it accidentally, when the mistake doesn't call attention to itself. If you are writing a conventional narrative, however, where you don't want to call attention to the writing as writing, then you should avoid this, since it will tend to break suspension of disbelief (and come across as an amateur mistake).

I note from your comments that your main character is actually a modern time traveler, alien to the historical setting. This is certainly one way to begin cuing or foreshadowing that fact, but it's a pretty blatant one, so if you really want the reader to be surprised, you'd have to go easy on it. Assuming you are writing from first person or close third-person perspective with this character as the viewpoint character, it will be very difficult to pull of a deception of this magnitude --at least if you play fair with the reader.

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There are some anachronisms in Shakespeare's work, and if Shakespeare can do it so can you.

In Julius Caesar, the clock strikes three, although clocks wouldn’t strike anything for another thousand years or more.
Hamlet was a student at Wittenberg University, which was less than a century old when the play was written, despite Shakespeare setting it centuries earlier.
Cleopatra decides to play billiards more than a thousand years before anything that resembled the game.

Source

Almost all of Shakespeare’s works are spiced with a moderate amount of anachronisms. The thing with his anachronisms though is that they are not mistakes on his part. More often than not, Shakespeare introduced them purposely for “dramatic purposes”. This means that he was very much aware that he was writing an anachronism, but put them in specifically in order to produce a special artistic effect that attracts the attention of the readers and the viewers of his plays.
Source

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I'd like to add a thing to all the already given answers.

It's definitely not ok if this feature is used only once in the whole story, but if it is consistent instead...

Chapter 3: The majestic sword.

[...] Edward's sword was black and shiny, like a brand new car. [...]

(I, as a reader, would think " ... wow, that's pretty random")

Chapter 4: The woods.

[...] The man was hiding in a dense forest. Edward was able to find him by cutting the bushes with his new sword, as sharp as a swiss army knife. [...]

(I, as a reader, would think " ... he did it again??")

Chapter 5: The castle.

[...] Edward reached the castle. It was immense: as wide as a small town, as tall as a skyscraper. [...]

(I, as a reader, would think " ... maybe this Edward actually knows something from our days")

My point is that, if you have a reason to do this (I read through some comments, is maybe the narrator unknowingly from the future?), then it can be used as a feature of your writing style and as a hint that at least one character is from the future. The style could lead to "oh, that's the writer that uses modern-day references in his medieval stories!", but only if it is a systematic thing.

If it's only for the sake of writing something unconventional, then it does not make sense: the reader has to be repaid by his discovery if that was the intent.

PS: I would make sure that everytime this element is used, it is clear that the character is the subject of the comment. Not necessarily by first-person narration, but just putting it as his thought.

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Assuming the entire story takes place in this historical setting: no. The story needs to work in absence of the future.

Technically speaking, there is no difference between a true medieval setting, and a fantasy medieval setting. Neither knows what a car is, and therefore you cannot reference a car.

Exception:
If someone from the present has traveled to the past; then you can obviously write their observations from their modern mindset.

There is an interesting thing to note here, which I think relates to your suggestion: Language is exempt from this rule and can be modernized.

The TV show Deadwood is a great example here. It's a very gritty, realistic show with a massive densitiy of expletives in the script (I think it still holds the title of most average "fuck"s per minute across multiple episodes).
Initially, the script was written with time-appropriate expletives. The problem was that time-appropriate expletives were things like "gosh", "darn", ...

This created an issue: while they were being historically accurate, an modern viewer would be amused by what is supposed to be an actual improper swear word.

If you say "Gosh, you're a darn thief!" to an 1850's person, that's the same as saying "God damnit, you're a fucking thief!" to a person today. It's strongly worded.
However, when you say "Gosh, you're a darn thief!" to a person today, it sounds tongue in cheek and completely loses the verbal force that is intended.

The writers decided to modernize the expletives, so that the meaning (in context) was understandable for viewer, even if the words were not factually correct.

For the same reason, we don't write medieval stories in Middle English (or whatever was appropriate at the time). While increasing the factual correctness, it dramatically lowers the readability of the script. One does not always outweigh the other.

There is no one true answer to this question. There is a spectrum of options:

Most media simply uses modern English, and omits referring to translation difficulties. This usually happens for stories that take place in scope of a single (main) language.
Some media chooses to retain the different language. Narcos, for example, does not shy away from having the majority of its show in Spanish even though it's targeted at a US audience. It's a matter of making things as realistic as possible (since they also used real footage inbetween the show's scenes).
Doctor Who predominantly uses modern English, and tries to provide a simple explanation (the main characters are under the effect of a piece of advanced technology that translates the world around them, including written language). This is used to a varying degree, and with varying outcomes: some cultural references are intentionally not understood by a foreign character (to remind the viewer of the translation mind shield), but at other times, idioms and saying do translate correctly. It's hard to keep it consistent.
There are cases where two different languages are both spoken in English, but in a different accent. In Allo! Allo!, the British speak with a stereotypically posh English accent, and the French speak English with a thick accent. In-universe, they could not understand each other (unless someone was known to be bilingual). There was one bilingual character (Michelle), who actually swapped between English and French accents when she talked to different people.
The Hunt for Red October has the first scene start in Russian. This carries over to Sean Connery's first few lines, and the viewer feels like they can't follow the story. But during the scene, they suddenly shift to speaking English. Because they are continuing the same conversation, the transition feels very natural. From that point on, they keep speaking English. During later scenes where the Russians and Americans communicate, they cannot understand each other (even though the viewer hears them speaking English).
Vikings handles this interestingly. It uses modern English as the conversational language, but this refers to different languages, based on the scene's context.

If two vikings are talking to each other, the actors speak modern English. If they happen upon an English character, that character will speak Middle English (factually correct).
If two English characters are talking to each other, the actors speak modern English. If they happen upon an viking, that viking will speak old Norse (factually correct) and they will not understand each other.
Languages can shift during a scene, if some characters leave the scene and new ones enter. The shift happens very similar to The Hunt for Red October, where they exchange a few lines in the foreign language and then shift to modern English.

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Attempting To Keep Readers' Minds Inside Our Story

As authors we attempt to do everything we can to keep our readers' minds in our story.

In most stories, as authors, we also want to disappear so the reader can forget she is even reading. In order to do that we create a setting such as 1400s England or whenever/wherever. We do our best to describe the setting so the reader isn't confused.

All of that work we do would be broken in the moment that the reader reads the sentence comparing the shiny sword to a shiny car.

But there is another explanation too.

Writing In The Perspective of A Character

The story should be written in the perspective of some viewpoint character. For example an author may use first-person viewpoint. And, obviously, in that case the first-person character (who lives in the 1400s) cannot make this analogy of shiny car to shiny sword.

But even if we used 3rd person close type of viewpoint we wouldn't want to make that kind of comparison because again the viewpoint character is living in the 1400s.

The main reason we don't do this is because it will tend to jar the reader from the story and make her question if the setting really is the 1400s since the author or character has mentioned an automobile.

As much as possible we do not want to jar the reader from the story because those are the moments when the reader may feel confused and may decide to stop reading.

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No, because you'd want your story to be as realistic and logical to the timeline your story is in. If you re-read that sentence, it would sound very weird because cars and swords were not used in the same time. Doing so will confuse the reader.

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