As or Like

While as and as if are conjunctions, like is a preposition: As you know, It’s as if you were lying, You look like a clown. A causal as that appears mid-sentence can be confused to mean while and should thus be avoided.

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 requires like to function as a preposition, with the object of a prepositional clause taking the objective case: He likes us, We are like them.