The nine most commonly used prepositions: at, by, for, from, in, of, on, to, with.

Note: Many of these words have other uses (adverbs or conjunctions, for example). They are prepositions only when followed by a noun or pronoun.

Examples:

to go--to + verb is an infinitive.

to school--to + noun is a preposition.

since starting back to college, John ...--since is a subordinating conjunction.

since yesterday--since is a preposition.

Forgive them, for they know not what they do--for is a coordinating conjunction.

for you--for + a pronoun is a preposition.

Jane ran across the street--across is a preposition.

We ran across my teacher at the supermarket--across works as an adverb.