How do I differentiate between kariru and kasu?

Though it's said that kariru is borrow and kasu is lend, I think it's used differently from english.

I lent him a book
I borrowed the book to him

I think it's essentially the same except for questions in which you use "kashite" Can lend me the book....? Hon wo kashite kuremasenka?

whereas in English we would normally say "May I borrow the book?"

but I lent him the book is still "Kare ni hon wo kashite agemashita" (in the case of a favor, or just "kashimashita."

and I borrowed the book from him is "Kare kara/ni hon wo karite moraimashita."

interesting. i never really thought about it, they both have different kanji, therefore they are 2 different words. i'll have to go away and look into the usage now!