Questions & Answers about Ключ застрял, поэтому я хлопнул дверью и вздохнул.
How would adding the particle было change the meaning of this sentence?
If you added было after a verb, like я хлопнул было дверью, it would turn it into a thwarted action. It would mean "I was about to slam the door (but stopped myself)" or "I slammed the door, but immediately reversed the action or regretted it." The particle было shows an action was intended or briefly started, but then interrupted.
Why is дверь in the instrumental case (дверью) here?
The verb хлопнуть (to slam or bang) requires the instrumental case for the object being slammed. You can think of it literally as making a loud noise with the door.
Хлопнул and вздохнул both have the -ну- suffix. Is this a coincidence?
No, this suffix is a common way to form perfective verbs that describe a single, sudden, or instantaneous action. It emphasizes that there was exactly one quick slam and one distinct sigh.
Could I use так что instead of поэтому for "so"?
Yes, you can use так что here. Both mean "so" or "therefore" in this context. Поэтому translates more literally to "for this reason," while так что focuses purely on the result, but they are completely interchangeable in this sentence.