“Like,” when it’s not busy being a Verb, is a Preposition.  “As,” on the other hand, is a Correlative Conjunction. 

For instance:  Her breath smelled like a dead fish, but when I told her so, she acted as if I had said something really bad.

Notice that “like” works just as any other preposition, and “as” works like you would expect a conjunction to act (which is the exact opposite of how I just used them).  “Like” should be followed by a simple noun phrase, and “As” should be followed by a more complex verb phrase. 

I  suppose that’s something you could learn, but when you do, who’s going to care?  It’s not like I really would.