native Slovak but cant explain :)
Hi there,
the question is, how should I explain "sa" and "si" in Slovak grammar, I mean there is "učím sa" not učím si and "myslím si" not myslim sa...
It so difficult for me to explain grammar rules :-/

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Answer

this could be useful 
http://www.answers.com/Slovak%20Language

no ale skus to vysvetlit takej cinanke...

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The rule is simple:

"sa" is accusative = Vidím sa v zrkadle (vidím koho/čo)
"si" is dative = Dám si pivo (dám pivo komu/čomu)

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Well, I am a native Slovak as well. I guess or the only rule I can think of right now is: We use "si" with the verbs like think - myslieť; wish - priať, želať; but also with verbs that imply that you do something for yourself or on your own such as „We are going to have a party“ – „Urobíme si párty“; And something more confusing with difference in meaning: „Look it up in a dictionary (for yourself)!“ – „Pozri si to v slovníku!“ versus „Look at the sky!“ – „Pozri sa na oblohu!“

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hum...i know! juts slike   i and y right? 
aye!its really not easy to learn!~ :(

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