Learn Croatian: Jesi li gladan?

In Croatian, you cannot start a sentence with the short, unstressed form of a verb like si. To form a yes/no question, you must use the full, emphatic form jesi at the very beginning of the sentence.

What does the little word li do?

The word li is a question particle. When you make a yes/no question by putting the full verb at the start of the sentence, you place li directly after it to clearly signal that you are asking a question.

Where is the word for "you" in this sentence?

It is built into the verb! The word jesi specifically means "you are" (for one person, informally). Croatian usually drops pronouns like ti (you) because the verb itself already tells us who is being talked about.

Would I use this exact sentence if I am speaking to a woman?

No, you would need to change the adjective. The word gladan is the masculine form, used when asking a man. If you are speaking to a woman, you must use the feminine form and ask Jesi li gladna?.