What's something in English that really surprised you?

I'm not sure I've ever hear "that's interesting" mean anything other than literally "that's interesting". Unless of course you say it in a sarcastic tone but you can do that with every phrase in English. The sarcastic tone may be what you're talking about but I just want to be clear that this is not special of the phrase "that's interesting" and can flip the meaning of any phrase in English.

The plural of beef is beeves

The "i before e, but e after c" rule. Blew my mind when I realized I'd been spelling words wrong my whole life

Nice Plane, one of my favorite ways to find out about these things from a foreign perspective is to watch stand-up by Ismo. https://youtu.be/1P0Z1yq-2FQ?si=j5goSbCX5G6MMqWw

For a European language, I'm very surprised how little conjugation English has. Add in no gender system, and it's very rare compared to the rest of the Indo-European language tree. Also, it's surprising how the accent can change on a word depending if it's the noun or verb, despite the same spelling. "Help me record this album?" vs "Let me play some music, I'll put on a record"

As a native speaker I was shocked to learn English has 4 conditionals

Does your native language not have sarcasm?

As an American, I was always aware of the differences that existed in the UK, but what I didn’t realize until some time later was that there are differences in punctuation rules between American and UK English as well.

Despite all of England's protests that the French are a collection of revolting, frog-loving, dirty-peasants, English sure does seem to have an aweful lot of French words. England and France sitting in a tree, K I S S I N G, first comes love, then comes marriage, then comes the rebirth of the Angevin Empire.

Latin plurals. The phrasal verb 'to make do'. The atrocious inconsistency of spelling vs. pronunciation, the first place belonging to 'choir'. The amazing (not in a good way) diversity of dialects. And the haunting feeling that English is closer to programming languages than the ones that people speak

I assume your example of someone saying "That's interesting" is a sarcastic tone? If it's sarcasm, the words don't really matter. The tone itself is the meaning. And that's not really specific to English.

Simple. Every sentence the person speaks is a lie.

How negative statements are agreed with in the negative rather than the affirmative. For example, if in English you asked "So no one cheated in that last exam?", if no one cheated you'd respond "no" and if someone had cheated you'd respond "yes". But for speakers of other European languages, the natural tendency is to respond "yes" if no one cheated (as if to say "yes, what you said is correct") and "no" if someone cheated (as if to say "no, that's incorrect").

The rule in American English about commas and periods inside quotation marks. Every time I saw it (for a very long period of time) I was absolutely sure it's just a typo. edit: oh, forgot about "1 in the morning"

A moment I always think about was with a student when I tutored English - he told me I was hot. I thought that was inappropriate and asked what he meant. He said I was friendly and made him feel comfortable. I asked, “do you mean warm?” And that is in fact what he meant. It’s funny how two words can have similar meanings (hot and warm), but if you use them interchangeably in the wrong setting, there’s a huge difference. Another student actually taught me about the concept of phrasal verbs. Obviously I’m familiar with them because I’m a native English speaker, but I never knew what they were called. She was so confused by the many, many uses of the word “get” which is something I had never considered before she mentioned it to me.

I see people saying that a lot, are there languages where there is no sarcasm?

The fact that both Kris' and Kris's are correct but not parents's

The use of "anyway". I couldn't tell an English learner what it means.

Probably when I, a native speaker, learned that English has an informal system for adjective order that everyone more or less agrees with and adheres to, often without consciously acknowledging it. My neighbor is a tall happy old German man. He is definitely not a German old happy tall man. And everyone just sort of... gets that? Crazy.

The expression “Now, then.”  Has absolutely nothing whatsoever to do with time.  You mostly use it when you’re turning from one task to another.  For example, say it’s your first day teaching a class.  You start by explaining where the restrooms are, what happens in the event of a fire alarm, and how much of the grade is quizzes and participation in discussions in class.  Then you open the textbook and say, “Now, then.”  And then you start introducing the class to the actual topic.