i cant understand when i should use diye and when i should use ki for me its the same for example in this 2 sentences

kopya yapasin diye kitabini acik birakti

sabun yok ki, yuzume yakayim

no one ?????

 There are just subtle differences.

The second one sounds more like an indignant response to the question why didnt you wash your face. You could just say "sabun yok". Adding the "ki" is like well, there wasn´t any soap, so how could I?

-sin diye is more like in order that, or so that, one thing can happen someone did something else.

kopya çeksin diye ... so that she could cheat

kitabýný açýk býraktý ... he left his book open

thank you very much .