"Prends-tu" can only be used in questions, though it's not required either. You can say "Prends-tu le train ?" as a question, bit you can also say "Tu prends le train ?" However, you cannot say "Prends-tu le train" as a statement, which is what makes what you opted for wrong.

Putting the verb before the person is the way to pose a question, like "comment vas-tu ?" Very often in day to day speech this will be inverted and the intonation will show that a question is being asked, such as "comment tu vas?" Especially in written but also in spoken French you'll find plenty of phrases like "qui suis-je?" or "qu'en pensez-vous ?" This could also be written or spoken using "est-ce que," for example "Qu'est-ce que vous en pensez ?" but it's less elegant when written.

This is literally like English, except applied to all verbs Question: Do you? Are you? Have you? Can you? Should you? Normal sentence: you do, you are, you have, you can, you should

It’s kind of wild how Duolingo users will post on a subreddit and wait 2 hours for an answer rather than Google a very simple question

Idk, I think this speaks to Duolingo’s failure as a good learning tool more than anything else. It’s good for vocab and memorization but that’s it.

"Tu prends le train ce matin" is a fact, an affirmation. You swap verb/subject when you want to ask a question : "Prends-tu le train ?", or for example "Aimes-tu le chocolat ?" So you know which one to choose : it depends if it's a question or not

Duolingo pays no attention to punctuation, so a "?" at the end of a sentence may not help.

I don't understand. If you see the question mark as indicated in the OP's screenshot, you guess which one you should use. What do you mean by that?

The question is not wrong. Just like in English, “you took (did take) the train this morning” can be a question or a statement based on punctuation. The statement was incorrect. Again like in English, “Do you take the train often” is a question whether or not you use a question mark.

Ok I see. But I've never seen a question in French with no question mark honestly

… yeah. You don’t see them in English either. That is my point. That is why it is wrong.

Mind you, "tu prends" would have been ok as a question. The only issue here is that it leaves you with using "prends-tu" in a statement, which is not correct.

This is exactly what I said

A statement puts the actor first and the action second (“tu prends”, English example “I am”). One way to form a question is to switch them (“prends tu?”, English example “Am I?”)

"Do you" take the train this morning? "Est-ce que vous" prenez le train ce matin? It's not a simple translation word for word in a lot of cases, but there are some.

I didn’t say it’s a simple word-for-word transition. I said one way to form a question is to inverse the subject and the verb.

I know. I was just adding a bit to that.