Speakers of non-English languages, what common mistakes do native speakers make that drive you crazy?

In Korean we have these conjugated forms. They both sound the same:

1. 나아 [na.a] (from 낫다) be/become better
2. 낳아 [na.a] (from 낳다) give birth (to a baby)

So when given A as an example:

(A) 감기에 걸렸어요. I got a cold.
(B) 빨리 나으세요! Hope you get better soon!
(C) 빨리 낳으세요! Hope you give birth soon!

For some reason Koreans across all ages write C instead of B by mistake. It became a national joke at this point and some do it ironically on purpose. I used to teach Korean. Imagine my face every time.

There are more but I'm on my phone. Will do more later.

I'm Spanish, n and ñ are different letters. They are not substitutes. It is the difference between someone being 5 years old and someone having 5 anuses.

"Yo tengo 5 años / yo tengo 5 anos"

Looking at you, Will Shortz

In Spanish, we have these words:

hay (there is) ahí (over there) ay (ouch)

And it's infuriating when people can't pick the right one in writing.

Less. It's used eveywhere, although should only be used with uncountable nouns.

Less drama is prefered.

Fewer items left on the shopping list.

In German people tend to increase "only" (das einzige). As in, they say something is the "onliest" (das einzigste). It's usually a good indicator of someone's education.

In many regions it is common to do comparisons with "as" (wie). As in "My dog is bigger as yours" instead of "My dog is bigger than yours". The most infuriating thing about this is that most people doing that mistake don't even acknowledge that it is one. At least people who say "onliest" can be convinced that it is wrong.

Technically not an error but still annoying is to append an apostrophe and an s to a name to indicate the genitive. Like in "Anna's food is good". In German that should be written as "Annas Essen ist gut". But due to many people making the same mistake (I guess also because we're used to it from English sentences) it has been allowed to use an apostrophe. So in that case I'm just a grumpy old guy.

I know you're asking for such errors in other languages, but I find it interesting that some of the common english errors are more frequent with native english speakers than with learners of english as a second language.

A good example of that is using "of" instead of "have".
Should of... of what?? It makes no sense to me how someone could confuse the two.

Having learned english as a second language, I learned to read and write it before learning to speak it.
On the other hand, I'd expect native speakers to have learned spoken english before learning written english.
I think this difference changes which errors someone is likely to make.
Native speakers confuse of/have more because they heard it long before writing it.
People who learned it later are less likely to make that mistake, although they're more likely for some others.

TL;DR: Native speakers are more likely to make mistakes that are homonyms. Of/have, your/you're, etc.

As for the spirit of your question, I'll go with french.

Almost every noun in french is gendered.
Objects, body parts, concepts, ideas, pretty much anything and everything is gendered.
It's also super obvious whenever someone doesn't use the correct gender for anything.
It's also hard to explain to anyone.
There might be a logic behind it, but I don't know how to summarize any of it.
I just know it, but couldn't tell you why.

Some of those make no fucking sense either.
It has mostly nothing to do with women or men or gebder roles and identity, it just is.
"Jam" is a feminine noun, yet "butter" is masculine.
"Bread" is masculine, but a "loaf" is feminine.
The noun for each and every season are masculine nouns, but the word "season" itself is a feminine noun.
Also, a "vagina" is a masculine noun, because reasons? Weird.
Various different words for "testicles" vary between masculine and feminine.

It's all super obvious to anyone who speaks french, but I never managed to explain it to any speaker of a non-gendered language like english without breaking their minds.

I really hate when native English speakers use could of or would of. It makes no sense and sounds completely wrong, yet some people claim it's just a minor mistake.

That English natives have so much trouble distinguishing effect from affect keeps surprising me.

As for Dutch, the dt-issue is presented as if it is this hugely complicated set of rules. While in reality it is dead simple. Third person in the present time is ALWAYS conjugated as stem+t for regular verbs, except in ONE case: when the stem already ends in t. Dt isn't special, it's just the rule applied to all stems.

I'm a native French speaker, specifically from the Acadian parts of the province of New-Brunswick (Canada). We have a lot of vocabulary, grammar and syntax that people who speak a more standard French might frown upon (lots of borrowing from English but also a lot of old French words which disappeared in Europe but not here, as well as some Indigenous influences). Fuck anyone who judges our dialect and accents, I love the way we speak.

That being said, there are a few things that bother me:

1. The pleonasm "plus pire" (most worst, or most most bad). There are a few common pleonasm but this one is the only one that truly irks me for some reason.
2. "Si que" (if that) because of something that was drilled into me by my dad, "les si n'aiment pas les que" ("the ifs don't like the thats"). Using "si que" is like saying "if that I say this" rather than "if I say this".

The more I think about it the more I guess my stance on this is that deviating from standard French is fine and even cool when it adds meaning or nuance. I just dislike it when it's purely redundant.

As a French speaker, I'm not mad at foreigners for not speaking French. I'm very tolerant for all their mistakes and I will help them if they want to.

I'm mad at French speakers mistakes though. Like people mixing first person futur and imparfait. Or people saying digital instead of numérique (those ones I hate them).

Due to Linguistics I spend more time trying to analyse the feature than judging it.

That said, two things that grind my gears, when it comes to Portuguese:

• Usage of the gerund for the future tense; e.g. estaremos enviando (roughly, "we will send") instead of "vamos enviar" or "enviaremos". My issue here is not grammatical, but that this construction usually marks lack of commitment.
• "Cuspido e escarrado" (spat and coughed up) to highlight the striking resemblance between two things or people. When the saying is supposed to be "esculpido em Carrara" (sculpted in Carrara).

I'm a native speaker of Mandarin Chinese from Taiwan. Some people often mix up 在 (zài) and 再 (zài) in writing. It's a bit hard to explain their definitions since they are merely function words (words that have little lexical meaning and express grammatical relationships among other words within a sentence), so I'm just gonna copy and paste their definitions from an online dictionary:

在: to exist; to be alive / (of sb or sth) to be (located) at / (used before a verb to indicate an action in progress)
再: again; once more; re- / second; another / then (after sth, and not until then) / no matter how ... (followed by an adjective or verb, and then (usually) 也 (yě) or 都 (dōu) for emphasis)

As you probably have noticed, their meanings don't overlap at all. The only reason some people mix them up is because they are homophones.

Another typo some... let's just say, less educated, people often make is 因該 (yīn'gāi). The correct word is 應該 (yīnggāi), meaning should; must. 因該 is never correct. You can think of 因該 as the Chinese version of the much dreaded "should of." The reason is that the distinction of -in and -ing is slowly fading away in Taiwan (it is still very much thriving in other Chinese-speaking societies), and some people just type too sloppily to care.

By the way, I should mention that 在, 再, and 應該 are very basic words, probably one of the first 500 words a non-native speaker learns.

Loose and lose. I just don't get it. I can understand when the words sound the same, like with the yours and the theirs but Loose and Lose don't sound the same. Like reading loose out loud in those sentences just sounds stupid.

In Swedish people often confuse de/dem(they/them kind of) and I honestly don't know exactly when to differentiate. You often learn to replace the word with another like vi/oss(we/us) to see if the sentence still sounds good and then you know the form you should use

Interesting question! Mandatory note that natural languages by definition aren't designed, and are always evolving, so if enough native speakers do something it is correct.

native speakers cannot by definition make systematic errors. they cannot make "common mistakes". if a thing is common, that's the correct way to say it. so what do you mean? spelling mistakes? (spelling is a separate thing from language)