The English "sit down" is being translated variously as leül, felül, and ül. Is there some rule for which to use when?
When it is a normal altitude for sitting (chair height), you use "leül". Anything higher than that, you use "felül".
'Ki leül a bútorokra' was marked wrong, though.
Because ' ki ül le a bútorokra?' is the correct form. The preverb is written in another word when it is a question or a negation.
There is even 'ráül' which is used in situations when you sit down onto something unusual place or thing-on the table, on an apple.