Questions & Answers about Когда идёт дождь, я спускаюсь в переход и иду к метро.
This is a very common Russian expression. Идёт дождь literally looks like the rain is going, but idiomatically it means it is raining.
Russian often uses verbs of motion in weather expressions:
- идёт дождь — it is raining
- идёт снег — it is snowing
So this is not about rain physically walking somewhere; it is just the normal Russian way to describe rainfall.
Когда here means when in the sense of whenever or at times when.
So:
- Когда идёт дождь... = When/Whenever it rains...
If you used если, the meaning would shift toward if:
- Если идёт дождь... = If it is raining...
Both can be possible in some contexts, but когда is natural here because the sentence describes a habitual situation: whenever it rains, I do this.