1) Sometimes, 学 and 学习 are the same.They're both verbs.You can use either of them to explain the meaning of learning something. 2) Sometimes, 学 and 学习 are different. 学 is only a verb. But 学习 can be a vi (viintransitive verb ) as well. For example, you can say 我在学习 or just 我学习 to show that you are studying.But you can't use 我在学 or 我学 to show the same meaning. Because 我在学 or 我学 should be used following a sentence which has indicated that what you are learning right now. 3) 念书 is only a vi (viintransitive verb ).It means "Study". You can say 我在念书 or 我念书，but you can't say 我念书中文, because 中文 is a noun. A noun cannot be used following a vi (viintransitive verb ). 念书 only can be used to show that you're studying. And what's more, this "Study" only means studying in schools. :) Good luck.