Questions & Answers about Не успели мы выйти за порог, как грянул ливень.
The translation says "than", so why is the Russian word как instead of чем?
This is part of the fixed construction не успел [infinitive] как [past tense] being taught in this lesson. It literally translates to "We hadn't managed to step out, how a downpour struck." When you want to say "no sooner had... than..." or "hardly had... when..." in Russian, you must use как to introduce the sudden second event.
Why is the infinitive выйти perfective here?
The verb успеть (to have time, to manage) almost always takes a perfective infinitive. This is because it focuses on achieving a specific, completed result—in this case, the single successful action of stepping out the door.
Why do we use за + accusative (за порог) here instead of another case?
The verb выйти implies directional motion. You are stepping across or beyond a boundary to the outside. The preposition за followed by the accusative case is used to show motion directed behind or beyond an object (the threshold).
Is грянул just another word for "started"?
It is much more dramatic than a basic word like начался (started). Грянуть means to burst forth, crash, or strike suddenly and loudly. It is typically used with powerful, sudden events like a downpour (ливень), thunder (гром), a storm (буря), or even a sudden burst of applause.