A preposition is a word that indicates location (in, near, beside) or some other relationship (about, after, besides) between a noun or pronoun and other parts of the sentence. A preposition isn't a preposition unless it goes with a related noun or pronoun, called the object of the preposition.
Let's meet before noon.
Before is a preposition; noon is its object.
We've never met before.
There is no object; before is an adverb modifying met.
Rule 1. A preposition generally, but not always, goes before its noun or pronoun. One of the undying myths of English grammar is that you may not end a sentence with a preposition. But look at the first example that follows. No one should feel compelled to say, or even write, That is something with which I cannot agree. Just do not use extra prepositions when the meaning is clear without them.
Correct: That is something I cannot agree with.
Correct: Where did you get this?
Incorrect: Where did you get this at?
Correct: How many of you can I depend on?
Correct: Where did he go?
Incorrect: Where did he go to?
Rule 2a. The preposition like means "similar to" or "similarly to." It should be followed by an object of the preposition (noun, pronoun, noun phrase), not by a subject and verb. Rule of thumb: Avoid like when a verb is involved.
You look like your mother.
That is, you look similar to her. (Mother is the object of the preposition like.)
You look like your mother does.
(Avoid like with noun + verb.)
Rule 2b. Instead of like, use as, as if, as though, or the way when following a comparison with a subject and verb.
Correct: You look the way your mother does.
Incorrect: Do like I ask. (No one would say Do similarly to I ask.)
Correct: Do as I ask.
Incorrect: You look like you're angry.
Correct: You look as if you're angry. (OR as though)
Some speakers and writers, to avoid embarrassment, use as when they mean like. The following incorrect sentence came from a grammar guide:
Incorrect: They are considered as any other English words.
Correct: They are considered as any other English words would be.
Correct: They are considered to be like any other English words.
Remember: like means "similar to" or "similarly to"; as means "in the same manner that." Rule of thumb: Do not use as unless there is a verb involved.
Incorrect: I, as most people, try to use good grammar.
Correct: I, like most people, try to use good grammar.
Correct: I, as most people do, try to use good grammar.
The rule distinguishing like from as, as if, as though, and the way is increasingly ignored, but English purists still insist upon it.
Rule 3. The preposition of should never be used in place of the helping verb have.
Correct: I should have done it.
Incorrect: I should of done it.
Rule 4. Follow different with the preposition from. Things differ from other things; avoid different than.
Incorrect: You're different than I am.
Correct: You're different from me.
Rule 5. Use into rather than in to express motion toward something. Use in to tell the location.
Correct: I swam in the pool.
Correct: I walked into the house.
Correct: I looked into the matter.
Incorrect: I dived in the water.
Correct: I dived into the water.
Incorrect: Throw it in the trash.
Correct: Throw it into the trash.