«Они́ в тебя́ ве́рят?»
Just one question... Is the "Do" really needed? Shouldn't "They believe you?" work as well?
Omitting 'do' in English is like omiting pronouns in Russian: it's acceptable and pretty common in colloquial speech, but generally considered a less 'correct' way of speaking. Duolingo generally doesn't allow such variants.
(I'm not a native speaker of English, so correct me if I'm wrong.)
I figured it was because Duolingo doesn't accept simplified colloquial speech. Though it is not slang, omitting "Do" is more commonly heard where I've lived; in the Mid-west.
It actually has a shade of different meaning:
-"Do they believe you?" is asking a yes/no question
-"They believe you?" is assuming the belief, and asking for confirmation or elaboration on why
I'm not sure exactly why, in this case... but this sentence seems to be triggering this interpretation for many of us English speakers.
I think either is acceptable and correct, "Do they believe you ?' maybe just a fraction more formal. 'Omitting' has two 't's though, to rhyme with 'committing '; not that English follows its phonetic rules very religiously, but '-iting' would rhyme with 'biting' or 'fighting ' .