In general to form a passive you combine a form of "be" with the participle of the verb. Now in the case of those two sentences: I like to write a letter to you. and He is trying to slap me. That would technically come out to: To write a letter to you is liked by me. and To slap me is tried by him. However, you should know these sound *very* strange to a native speaker. The principle reason is the use of "like to" and "try to" don't play well with the passive construction. Changing the infinitives to gerunds makes it sound a touch better. Writing a letter to you is liked by me. and Slapping me is tried by him.

1. To write a letter to you is liked by me. 2. To slap me is tried by him.