Consider the professor's desk and all the prepositional phrases we can use while talking about it.

You can sit before the desk (or in front of the desk). The professor can sit on the desk (when he's being informal) or behind the desk, and then his feet are under the desk or beneath the desk. He can stand beside the desk (meaning next to the desk), before the desk, between the desk and you, or even on the desk (if he's really strange). If he's clumsy, he can bump into the desk or try to walk through the desk (and stuff would fall off the desk). Passing his hands over the desk or resting his elbows upon the desk, he often looks across the desk and speaks of the desk or concerning the desk as if there were nothing else like the desk. Because he thinks of nothing except the desk, sometimes you wonder about the desk, what's in the desk, what he paid for the desk, and if he could live without the desk. You can walk toward the desk, to the desk, around the desk, by the desk, and even past the desk while he sits at the desk or leans against the desk.

All of this happens, of course, in time: during the class, before the class, until the class, throughout the class, after the class, etc. And the professor can sit there in a bad mood [another adverbial construction].

Those words in bold blue font are all prepositions. Some prepositions do other things besides locate in space or time — "My brother is like my father." "Everyone in the class except me got the answer." — but nearly all of them modify in one way or another. It is possible for a preposition phrase to act as a noun — "During a church service is not a good time to discuss picnic plans" or "In the South Pacific is where I long to be" — but this is seldom appropriate in formal or academic writing.

Is it any wonder that prepositions create such troubles for students for whom English is a second language? We say we are at the hospital to visit a friend who is in the hospital. We lie in bed but on the couch. We watch a film at the theater but on television. For native speakers, these little words present little difficulty, but try to learn another language, any other language, and you will quickly discover that prepositions are troublesome wherever you live and learn. This page contains some interesting (sometimes troublesome) prepositions with brief usage notes. To address all the potential difficulties with prepositions in idiomatic usage would require volumes, and the only way English language learners can begin to master the intricacies of preposition usage is through practice and paying close attention to speech and the written word. Keeping a good dictionary close at hand (to hand?) is an important first step.

Prepositions of Time: at, on, and in

We use at to designate specific times.
The train is due at 12:15 p.m.

We use on to designate days and dates.
My brother is coming on Monday.
We're having a party on the Fourth of July.

We use in for nonspecific times during a day, a month, a season, or a year.
She likes to jog in the morning.
It's too cold in winter to run outside.
He started the job in 1971.
He's going to quit in August.

Prepositions of Place: at, on, and in

We use at for specific addresses.
Grammar English lives at 55 Boretz Road in Durham.

We use on to designate names of streets, avenues, etc.
Her house is on Boretz Road.

And we use in for the names of land-areas (towns, counties, states, countries, and continents).
She lives in Durham.
Durham is in Windham County.
Windham County is in Connecticut.

Prepositions of Movement: to
and No Preposition

We use to in order to express movement toward a place.
They were driving to work together.
She's going to the dentist's office this morning.

Toward and towards are also helpful prepositions to express movement. These are simply variant spellings of the same word; use whichever sounds better to you.
We're moving toward the light.
This is a big step towards the project's completion.

With the words home, downtown, uptown, inside, outside, downstairs, upstairs, we use no preposition.
Grandma went upstairs
Grandpa went home.
They both went outside.

Prepositions of Time: for and since

We use for when we measure time (seconds, minutes, hours, days, months, years).
He held his breath for seven minutes.
She's lived there for seven years.
The British and Irish have been quarreling for seven centuries.

We use since with a specific date or time.
He's worked here since 1970.
She's been sitting in the waiting room since two-thirty.

Prepositions with Nouns, Adjectives, and Verbs.

Prepositions are sometimes so firmly wedded to other words that they have practically become one word. (In fact, in other languages, such as German, they would have become one word.) This occurs in three categories: nouns, adjectives, and verbs.

A combination of verb and preposition is called a phrasal verb. The word that is joined to the verb is then called a particle. Please refer to the brief section we have prepared on phrasal verbs for an explanation.

Idiomatic Expressions with Prepositions

• agree to a proposal, with a person, on a price, in principle
• argue about a matter, with a person, for or against a proposition
• compare to to show likenesses, with to show differences (sometimes similarities)
• correspond to a thing, with a person
• differ from an unlike thing, with a person
• live at an address, in a house or city, on a street, with other people

Unnecessary Prepositions

In everyday speech, we fall into some bad habits, using prepositions where they are not necessary. It would be a good idea to eliminate these words altogether, but we must be especially careful not to use them in formal, academic prose.

• She met up with the new coach in the hallway.
• The book fell off of the desk.
• He threw the book out of the window.
• She wouldn't let the cat inside of the house. [or use "in"]
• Where did they go to?
• Put the lamp in back of the couch. [use "behind" instead]
• Where is your college at?

Prepositions in Parallel Form

 When two words or phrases are used in parallel and require the same preposition to be idiomatically correct, the preposition does not have to be used twice.
You can wear that outfit in summer and in winter.
The female was both attracted by and distracted by the male's dance.

However, when the idiomatic use of phrases calls for different prepositions, we must be careful not to omit one of them.
The children were interested in and disgusted by the movie.
It was clear that this player could both contribute to and learn from every game he played.
He was fascinated by and enamored of this beguiling woman.