Learn Portuguese (Portugal): Prova a sopa antes de pôr sal.

Prova is the informal command (imperative) for tu (you). To give a positive command to a friend, you just drop the final s from the present tense tu provas. If you were speaking formally to você, the command would be prove.

Does provar mean "to prove" or "to taste"?

It means both! When talking about food and drinks, provar means to taste or to try. In other situations, it means to prove, like proving a fact.

Why is the verb pôr not conjugated here?

In Portuguese, when a verb comes right after a preposition like de (as in the phrase antes de), it stays in the infinitive (dictionary) form. While English uses the -ing form ("before adding"), Portuguese literally says "before to put".

Why does the sentence use pôr (to put) instead of a word for "to add"?

While you could use adicionar (to add), native Portuguese speakers usually just use pôr (to put) when talking about adding ingredients in everyday conversation.