In these days, the plague of “like” centers around its substitution for perfectly good verbs, such as “become” or “say.” This post assumes that you do not lapse into what used to be called “Valley-Girl-Speak,” along the lines of: “Well, I was like ‘yuck’ but he was all like ‘why’ and, like, I was all ‘whatever’.”
A half-century ago or more, R.J. Reynolds Tobacco Company came out with a promotion for Winston cigarettes that caused a stir in the circles of those who notice the premeditated use of bad syntax in advertising: “Winston tastes good, like a cigarette should.” The problem is that “like” is used as a subordinating conjunction. “Like” is not a conjunction; it is a preposition. The correct word to use is “as“: “Winston tastes good, as a cigarette should.”
Well, you can say that “like” has become a conjunction simply because so many people use it that way. After all, that is how language grows and changes. True, you will hear “like” used incorrectly by hordes of ignorant or reckless speakers. In informal speech, you will probably get away with it, at least if there are no Grammar Nazis around. But it is wrong, and you should know better, especially when putting pen to paper or speaking to those who themselves do know better.
As a preposition, “like” takes a noun or pronoun as an object: “She plays ragtime piano like a professional.” Notice that there is no verb in the phrase. It is not a clause. If you want to use a clause, then “as” is your word: “She plays ragtime piano, as any professional would play.”
One other thing: Because “like” is a preposition, it should never be used as a synonym for “as if,” which, like “as,” is a conjunction. If you hear yourself say, “I feel like I’m coming down with a cold,” stop. It’s better to say, “I think I’m coming down with a cold,” or “I feel as if I were coming down with a cold.” This second option uses the subjunctive mood because there is uncertainty about the statement. Also, it is a regional mannerism to utter “feel like” when you mean “think” as in “I feel like she should be more polite.” Perhaps your feelings are involved, but you are saying what you think, so be direct and explicit: “I think she should be more polite.”
In the same vein, it is perfectly correct to say, “I feel like a nut,” whether it means you crave a nut, or whether it means that a nutty feeling is overtaking you. You could say that in the case of a craving, the verb is really a phrasal verb: “feel like” (meaning to have a yen for). In the second case, the verb is “feel.” The prepositional phrase “like a nut” acts as an adverb to explain what you are feeling.
Of course, “like” can also be used as a verb, as in “I like coffee, and she likes tea”. That should present neither a problem nor a confusion. “Like” is sometimes used as a noun on Facebook: “How many ‘Likes’ did you get?” That nominalization, unique to Facebook, is not even arguably standard usage.
