Learn Russian: Кто ставил пылесос перед плитой? - Who put the vacuum cleaner in front of the stove?

This is a classic example of the annulled result. By using the imperfective ставил, the speaker implies that someone put the vacuum cleaner in front of the stove, but it is no longer there now. If the speaker had used the perfective поставил, it would mean the vacuum is still sitting right there.

Wait, doesn't ставил usually mean "was putting" or "used to put"?

Yes, the imperfective past is normally used for ongoing or repeated actions. However, with verbs of placement or state change (like putting, opening, or taking), using the imperfective for a single completed action specifically communicates an annulled result. The action happened and finished, but its result was reversed.

Why is перед плитой in the instrumental case? Doesn't "putting" imply motion, which usually takes the accusative?

That is a great catch! Normally, the destination of a motion takes the accusative case. However, the preposition перед (in front of) almost always takes the instrumental case, whether it is describing a static location or the destination of a movement.

Why do we use ставил for putting a vacuum cleaner, instead of another word for "put" like клал?

In Russian, how you put something depends on the object's orientation. You use ставить (to stand something up) for things that have a natural base or are meant to stand vertically, like a vacuum cleaner, a lamp, or a bottle. You use класть (to lay something down) for flat objects or things placed horizontally.