Learn Russian: Я не мог не вмешаться.

In Russian, combining не + a form of мочь (like the past tense мог) + не + an infinitive creates a strong affirmation of inevitability. Literally, it means "I was not able to not [do something]." English translates this naturally as "I couldn't help but..." or "I simply had to..."

Negated verbs often take the imperfective aspect. Why do we use the perfective вмешаться in this sentence?

Because this sentence isn't actually a negative statement in meaning—it's a strong positive statement ("I did intervene"). Since the speaker is referring to a single, specific, completed action in a particular situation, the perfective aspect вмешаться is required.

How does the prefix and suffix of вмешаться create the meaning "to intervene"?

The verb is built from the prefix в- (in/into) + мешать (to mix) + the reflexive suffix -ся (oneself). Literally, it means "to mix oneself into" a situation, which is a helpful way to remember that it means "to intervene" or "to interfere."

If a woman were speaking, would the sentence change?

Yes. Since мог is the past tense masculine form of мочь, a female speaker would need to use the feminine form могла, making the sentence: Я не могла не вмешаться.