Learn Croatian: Zanima ga je li šišanje jeftino. - He wonders if the haircut is cheap.

The verb zanimati literally means 'to interest'. In Croatian, instead of saying 'he is interested' or 'he wonders', you say 'it interests him'. The word ga is the accusative pronoun for 'him'. The entire reported question (je li šišanje jeftino) acts as the subject that interests him.

How does je li work here to mean 'if' or 'whether'?

In standard Croatian, indirect yes/no questions are introduced with je li. This replaces the 'if' or 'whether' you would use in English. While you will often hear people use da li in casual everyday speech, je li is the standard grammatical way to report a yes/no question.

Where is the verb for the second part of the sentence? Shouldn't there be an 'is' for 'is cheap'?

The je in je li is actually doing double duty here. It combines with li to introduce the indirect question, but it is also the actual verb 'is' (from biti). Because it is already there at the start of the clause, you don't add another je later in the sentence.

Why does the word for cheap (jeftino) end in -o?