Questions & Answers about Co robisz w czasie wolnym?
Why is it w czasie wolnym instead of w czas wolny?
The preposition w (in) requires the Locative case here. The basic dictionary phrase is czas wolny (free time). In the Locative case, the noun czas changes to czasie, and the adjective wolny changes its ending to match, becoming wolnym.
Is it wrong to say w wolnym czasie with the adjective first?
No, w wolnym czasie is also perfectly correct and very common. In Polish, adjectives often go after the noun when they define a specific category or set phrase (like free time as opposed to working time). However, putting the adjective before the noun is also fine in everyday speech.
Why isn't the word ty (you) in this sentence?
In Polish, you can usually leave out the subject pronoun because the verb ending does the work for you. The ending of the verb robisz already means you do. You would only add ty (Co ty robisz?) if you wanted to add strong emphasis, as in: What do YOU do?
Could I use the perfective verb zrobisz here instead of robisz?
Not in this context. Zrobisz is a perfective verb used for a single, completed action in the future. Because this sentence asks about a regular habit or ongoing routine, you need the imperfective verb robisz.