Learn Russian: Если бы он угрожал мне, я бы отомстил. - If he threatened me, I would take revenge.

The verb угрожать (to threaten) is a verb of communication that directly governs the Dative case without a preposition. Think of it as directing a threat to someone, which is why it is угрожал мне.

How do we form the "if , I would [y]" structure used here?

To talk about hypothetical or unreal situations, Russian uses the particle бы with the past tense in both halves of the sentence. The condition uses если бы + past tense (угрожал), and the result uses бы + past tense (я бы отомстил).

Why is угрожал imperfective, while отомстил is perfective?

Угрожал is imperfective because the threatening is viewed as a general action or ongoing behavior. Отомстил is perfective because taking revenge is envisioned as a single, decisive, and completed reaction.

If I wanted to say "I would take revenge on him," how would that work with отомстить?

The verb отомстить (to take revenge) also requires the Dative case without a preposition. To add "on him," you would simply use the Dative pronoun ему and say я бы отомстил ему.