Learn Russian: Кто бы тебя ни осуждал, не слушай их. - Whoever condemned you, do not listen to them.

The structure of a question word plus бы ни translates to 'no matter who' or 'whoever'. Here, Кто бы ни means 'whoever'. The ни isn't negating the verb; it acts as an intensifier to show the statement applies universally, under any condition.

The meaning feels like a general rule for the present or future, so why is the verb осуждал in the past tense?

The particle бы always requires the verb to be in the past tense, regardless of the actual time frame you are talking about. This is the same rule you learned for basic conditional sentences. Here, it creates a hypothetical or generalized scenario: 'whoever might condemn you'.

The pronoun Кто takes the singular verb осуждал. Why does the second half use the plural pronoun их to refer back to it?

Just like the 'singular they' in English, Russian speakers often use the plural их (them) when referring back to an unspecified Кто (whoever). It intuitively acknowledges that 'whoever condemns you' could be multiple people. Using the masculine singular его is also grammatically correct but less common in this specific context.

Why use the imperfective verb осуждал here instead of the perfective осудил?

Using the imperfective осуждал treats the condemnation as an ongoing or repeated possibility, meaning 'anyone who generally judges you'. If you used the perfective осудил, it would focus on one specific, completed act of condemnation.