Learn Croatian: Gdje je jakna? - Where is the jacket?

The word je is a short form of the verb to be (an enclitic). In Croatian, these short words are very strict about where they sit: they almost always have to take the exact second position in a sentence. Since Gdje is the first word, je naturally falls into the second spot.

Can I use the long form jest instead of je in this question?

We almost never do. When asking a question that starts with a question word like Gdje (where) or Tko (who), you should use the short form je. The long form jest is mostly saved for putting special emphasis on the verb or giving a strong affirmative answer.

How do we know it means "the jacket" and not "a jacket"?

Croatian does not have definite or indefinite articles like "the" or "a". The word jakna alone can mean "jacket", "a jacket", or "the jacket". You will always know which one is meant based on the context of the conversation.

Does the ending of jakna tell us its gender?

Yes! Nouns that end in -a in their basic dictionary form are almost always feminine in Croatian. Knowing this will be very helpful when you need to match adjectives to the noun later on.