Like is a preposition. It should be followed by an object to make a prepositional phrase. "She talks like a teacher." 'As' is a conjunction. It is followed by a clause with a subject and verb. "She talks as a teacher does." It is incorrect to say: "She talks like a teacher does.", even though some people use it. You can also use 'as' in a comparison. "She is as tall as Martha."

The proper way to differentiate between like and as is to use like when no verb follows. For example, John runs like a duck or He spoke just like his brother. Notice that when I use like, the words that come after are generally simple. A duck and his brother are the objects of the preposition. If the clause that comes next includes a verb, then you should use as. For example, John runs as if he only had one leg or She acted just as I would have expected. Notice that when I use as, the words that come after tend to be more complex. You generally hear like used in everyday speech, so that helps me remember that like is the simpler word—or at least it is followed by simpler words. As sounds stuffier and is followed by a more complex clause that contains a verb.