"What do you have in your pocket?"
Translation:Cos'hai in tasca?
The Italian "in" translates directly to the English "in".
The Italian "nel" is a contraction of the Italian "in+il", ie. means the English "in the" (so "nella"="in+la"="in the [feminine object]")
So in this question, "Cos'hai nella tasca" would be counted as incorrect, as Duo didn't ask for the article. However, both "Cos'hai in tasca" and "Cos'hai nella tua tasca" would be correct, as Duo (probably quite rightly) considers it implied that the pocket in question is yours.
You can! Cos'hai and cosa hai are both correct... as well as che cosa hai...che cos'hai... elision and troncamento give rise to a lot of confusion and discussion because many forms can be considered correct. Some are more appropriate in literature, others are more appropriate colloquially... the topic is quite dense.
I'm new and reading so many of your often comments. This is my first post. I'm beginning to think many of us are missing the point... Some are trying too hard to get DL to say their answer is correct. As soon as you are correct, you stop learning. When we're learning, first we learn ONE correct answer even though we later learn there are maybe 4 or more correct answers (depending on register, familiarity, formality, hip or old-fashioned etc.) I would think it BETTER if DL forced only the 'unexpected' answer "in tasca" to be considered correct. Only that way do we learners widen our own range of possible correct answers. Are we not all advanced learners in DL enough to have long ago stopped believing that the only correct Italian is when every English word translates to exactly one Italian word. We're not here on DL to try to change the Italian language to be more like English ;-)
I'm neither native nor an expert, but to those wondering/having asked, the difference between "in tasca" and "nella tua tasca" is idiomatic, is a question of normal language use, not of consistent rules. So you would have similar examples like "in cucina" "in macchina" "in treno"