preposition

Prepositions "On," "At," and "In"

A preposition is a word that links a noun, pronoun, or noun phrase to some other part of the sentence.
Prepositions can be tricky for English learners. There is no definite rule or formula for choosing a preposition. In the beginning stage of learning the language, you should try to identify a preposition when reading or listening in English and recognize its usage.
A preposition is used to show direction, location, or time, or to introduce an object.
Here are a few common prepositions and examples.

On

Used to express a surface of something:
I put an egg on the kitchen table.
The paper is on my desk.
Used to specify days and dates:
The garbage truck comes on Wednesdays.
I was born on the 14th day of June in 1988.
Used to indicate a device or machine, such as a phone or computer:
He is on the phone right now.
She has been on the computer since this morning..

At

Used to point out specific time:
I will meet you at 12 p.m.
The bus will stop here at 5:45a.m
Used to indicate a place:
There is a party at the club house.
There were thousand of people at the park.

In

Used for unspecific times during a day, month, season, year:
She always reads newspapers in the afternoon.
In the summer, we have a rainy season for three months.
The new semester will start in january

Common Verb and Preposition Combinations

Verbs and prepositions are also used in common phrases. The verb generally goes in front of the preposition.

Do you agree with the new guidelines?

Do you agree to the new guidelines?

You get what you pay for.

This novel gave me a lot to think about.

It’s easy to fall in love with big, silly dogs.

I have no doubt about the outcome of the vote.

Rock on, dude!

The verb and the preposition may be separated, especially if the sentence is formally written so the preposition isn’t at the end.

Didn’t I warn you about leaving things until the last minute?

To whom does this book belong? (Who does this book belong to?)

Could you please explain this math problem to me?

To explain something to someone is a common phrase which will always be separated by the something.
Some other examples of verb/preposition combinations are ask for, depend on, look after, wait for, rely on, dream on, lead on, do something for someone, be careful of, be careful with, be disappointed by, boast about, complain about, in conclusion, and search for.
 

 

 references :
http://www.talkenglish.com/grammar/prepositions-on-at-in.aspx
http://www.grammarly.com/handbook/grammar/prepositions/9/common-verb-and-preposition-combinations/