Learn Croatian: Da, jesam.

In Croatian, short verb forms like sam are weak words that cannot stand on their own or be emphasized. When giving a short answer like 'I am', you must use the full, emphatic form jesam.

Can I just say Da, sam.?

No. Because sam is a weak form, it always needs to lean on another word in a longer sentence. It can never be used by itself as a short answer.

Should I include the pronoun and say Da, ja jesam.?

It is not necessary. The verb jesam already means 'I am' all by itself. You would only add ja if you wanted to strongly emphasize that you are the one doing something, as opposed to someone else.

If someone asks me a question using a short form, do I still reply with this full form?

Yes! For example, if someone asks Jesi li dobro? (Are you well?), you answer with the full form Da, jesam. to say 'Yes, I am'.