Learn Croatian: Moram kupiti octa prije nego što ona dođe. - I have to buy some vinegar before she comes.

This is the bare partitive genitive, the focus of this lesson. Using the genitive octa on its own expresses an unspecified quantity, meaning 'some vinegar'. If you used the accusative ocat, it would mean you are buying 'the vinegar' (a specific bottle you already talked about) or vinegar in general.

The dictionary form for vinegar is ocat. Where did the 'a' go in octa?

This is an example of the 'fleeting a'. In many Croatian words, the letter a in the final syllable of the base form drops out when a case ending is added. So, ocat becomes octa in the genitive. You have likely seen this before with words like pas (dog) becoming psa.

Why do we need the whole phrase prije nego što instead of just using prije for 'before'?

You use prije on its own when it is a preposition followed by a noun, like prije posla (before work). But when 'before' introduces a full clause with a subject and a verb—in this case, ona dođe (she comes)—you must use the conjunction prije nego što.

Could I use the imperfective verb dolazi instead of the perfective dođe here?