"Do you understand what the teacher is talking about?"

Translation: 你懂老师在说什么吗？

[ChHN2]

Why is there a need for 在 in this sentence?

[JaniPatoka]

Because the teacher "is speaking", not "says".

[Cecil164832]

Yeah, but if I was in a class and whispering to my friend, I would not necessarily add the 在. In the context, it would be understood that I would be asking about what the teacher was talking about. Actually, I'd just ask, "老师说什么?"

[stephen_zissou]

Duolingo couldn't possibly allow for all variations in all contexts - it's hard enough to account for all of the strictly correct variations. So in this course, English present progressive seems to be correlated with 在 in front of the verb in Chinese. Just the way it has been in this course so far.

[em2283]

Why does the ma need to be added at the end

[Cinnamon5230]

It's a yes/no question. So you need to use 吗. If it's not, you won't need it.

This can lead to some tricky sentences for a Chinese beginner:

老师在说什么？ What does the teacher say?

老师在说什么吗？Is the teacher saying something?

[AmyLee489653]

To indicate that the speaker is asking a question.

[Clint506209]

It makes it a question. If you said it without the ma but used the right infection for a question, it would be understood as a question but has an implication that it is unexpected that you would understand.