Learn Croatian: Ako nema vune, kupila bih deblji materijal. - If there is no wool, I would buy a thicker material.

The base adjective is debeo (or debeli). To form the comparative, we take the root debel- and add the suffix -ji. This triggers a sound change called jotation, where the l and j merge into the single letter lj, creating deblji.

Why is the word for wool written as vune instead of vuna?

The word nema, when used to mean "there is no" or "there isn't", always forces the noun that follows it into the genitive case. Because vuna is a regular feminine noun, it takes the genitive ending -e.

How do we know the speaker of this sentence is female?

The phrase kupila bih means "I would buy". The active past participle kupila ends in -la, which is the feminine form. If a man were speaking, he would use the masculine form and say kupio bih.

Since deblji materijal is the object being bought, why doesn't it look like it is in the accusative case?

It actually is in the accusative case! Materijal is a masculine inanimate noun (an object, not a living being). For masculine inanimate nouns, the accusative form looks exactly the same as the nominative form, and the adjective deblji simply matches it.