Like vs. As and As IfHow to use "like," "as," and "as if" correctly
Like is a preposition, so it cannot take a clause (a group of words with a subject and verb) after it.
Like + clause:
felt like I was flying (I is the subject; was flying is the verb)
want to do that just like he does (he is the subject; does is the verb)
(Note, however, that in informal English like is very commonly used in such situations.)
Instead of like, use as or as if, which can act as subordinating conjunctions and take clauses after them. (As though is an equivalent alternative to as if.)
As or as if + clause:
felt as if I were flying (felt as though I were flying)
want to do that just as he does
However, like can be used before a noun or pronoun that is not the subject of a clause. This noun or pronoun is the object of the preposition:
felt like a taste explosion (explosion is the object of the preposition like)
want to be just like him (him is the object of the preposition like)
One simple rule to remember is that following like with a nominative case pronoun (such as he above) is almost always wrong. In such cases, either like is used incorrectly, or the nominative case is used incorrectly.
For more information on topics like clauses, prepositions, and pronoun cases, see the various grammar pages on this site. For more information on the proper use of like, see my Like vs. For Example page.
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage