As and as if are conjunctions; like is a preposition: I’m old, as you know; It’s as if you were lying; You look like a clown. Avoid the causal as (in place of because, since, for), since (not as) it can be confused for the temporal while.
Traditionally, like is adjectival, modifying nouns or pronouns, and as is adverbial, modifying verbs: Eyes like his, Cold as frost. Although like often replaces as and as if in colloquial use, proper usage still prefers like to function as a preposition, with the object of a prepositional clause taking the objective case: He is like us, We are like them.