"I will ask you only one question."
Translation:Я задам тебе только один вопрос.
As it does not carry the typical ending of я -у/-ю, what kind of form is задам?
«Задать» ‘to give, to prescribe’ is an irregular verb, it has -м instead of -у/ю. So, «я зада́м» means ‘I will give’ (it’s a perfective verb so задам is a future tense).
«Дать» ‘to give’ and verbs derived from it with other prefixes also have -м instead of -у/-ю.
A similar verb is «есть» ‘to eat’: я ем ‘I’m eating’ also has -м here.
«Спра́шиваю» is the imperfective verb, that's why it's basic form is not the future tense but the present tense. So, 'спра́шиваю' is 'I'm asking', not 'I will ask'. You need 'спрошу́' (the corresponding perfective verb) or 'бу́ду спра́шивать' (this adds a meaning of duration: probably I'll be asking it several times, or probably the question is so long that I'll be asking it for a noticeable period of time).
Then, «спра́шивать» uses different cases. «Зада́м» requires dative (зада́м тебе́): you might notice it's derived from «да́ть», so it uses the same case forms. «Спра́шивать» and «спроси́ть», on the other hand, require either Accusative (спрошу́ тебя) or «у» + Genitive (спрошу́ у тебя́).
The last and biggest problem is that «спроси́ть вопро́с» sounds too redundant. Since 'спроси́ть' already means 'ask a question', and it even has the same root (-прос-), we don't usually say «спроси́ть вопрос» or «спра́шивать вопро́с». The English verb 'ask' has many different meaning (ask for help, ask directions), so adding 'ask a question' helps. The Russian verb has only one meaning, so «спроси́ть вопро́с» is not used.
Without «вопро́с», you could use:
- Я спрошу́ (у) тебя́ то́лько об одно́м.
- Я спрошу́ у тебя́ то́лько одно́. (You can't use 2 accusatives: if you use Accusative for 'тебя', then you can't use Accusative for the thing being asked.)
These forms sound natural too, but they're too different from the English sentence grammatically, so probably Duolingo doesn't accept them (although I haven't tried; maybe it does?).
Sorry — I've edited my comment. Should probably go to bed now. ^^'