Learn Russian: Я должен попробовать соус, чтобы его не пересолить. - I must taste the sauce so that I don't oversalt it.

The prefix пере- is used to show that an action was done to excess. Since солить means "to salt", adding пере- gives us пересолить, meaning "to oversalt" or "to put too much salt in".

Why is the verb пересолить in the infinitive here? Doesn't чтобы usually require the past tense?

You only use the past tense after чтобы when the subject changes between the two clauses. Because the person tasting the sauce and the person potentially oversalting it are the exact same person (I), the second clause simply uses the infinitive.

Why is the pronoun его used for the sauce instead of он?

It is the direct object of the verb пересолить, so it must be in the accusative case. While the masculine noun соус is inanimate and doesn't change its ending in the accusative, the pronoun он always changes to его in the accusative case.

Why do we use the perfective verb попробовать here?

You are tasting the sauce as a single, completed action to check the seasoning right now. The perfective попробовать captures this one-time result, whereas the imperfective пробовать would imply an ongoing process or a repeated habit.