Is the following sentence acceptable? Which is better?
(1) He was grateful to me for my good deed.
(2) He was grateful to me for what I did for him.
(3) He deeply appreciated my good deed.
All are okay grammatically. The first is more specific about what you did. The third makes him more grateful.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.