Questions & Answers about Dev nehirden su içmiş.
Why use içmiş instead of içti here?
The suffix -miş is the reported or inferential past tense. You use it when you didn't see the action happen yourself, but you heard about it, saw evidence of it, or are telling a story. Since this sentence is about a giant (dev), it sounds like a fairy tale or myth. Turkish fairy tales almost always use -miş because the storyteller wasn't there to witness the events!
Why is there a -den added to nehir?
The suffix -den (or -dan) means 'from' or 'out of'. By attaching it to nehir (river), we get nehirden, which translates to 'from the river'.
Why is it just su and not suyu?
We leave su (water) in its plain form because it is a general, indefinite object here. The giant didn't drink 'the' specific water you were just talking about; he just drank 'water' in general. If he had drank that specific water, it would take the accusative case ending and become suyu.