MENJAWAB PERTANYAAN - Bahasa Indonesia

What's the difference between these sentences?

First case :

"Saya memakan apel kemarin" and "Saya sedang memakan apel."

Second case :

"Saya sedang memakan apel" and "Saya sedang makan apel"

Third case :

"Saya sedang makan apel" and "Saya makan apel"

Sorry for the bad english.

Actually, I'm native speaker of Bahasa  

Imho, not all of the native speakers of Bahasa can explain the difference :|