Hi Alisa,
I'll try to explain it from what I do know...
The reflexive (3. alebo 4. pád) in Slovak comes "as second as possible" in a sentence. So you'd never begin a sentence with "sa/si", but depending on the other words, the reflexive can be shuffled forward, before the verb. I'm still trying to get my head around it (especially when I use compound sentences and adverbs of time), but that seems to be the general idea.