English grammar: Prepositions ON IN AT

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

Used to specify days and dates:

• The garbage truck comes on Wednesdays.

Used to indicate a device or machine, such as a phone or computer:

• He is on the phone right now.

Used to indicate a part of the body:

• The stick hit me on my shoulder.

Used to indicate the state of something:

• The building is on fire.

At

Used to point out specific time:

• I will meet you at 12 p.m.

Used to indicate a place:

• There is a party at the club house.

Used to indicate an email address:

• Please email me at abc@defg.com.

Used to indicate an activity:

• He laughed at my acting.

In

Used for unspecific times during a day, month, season, year:

• She always reads newspapers in the morning.

Used to indicate a location or place:

• She looked me directly in the eyes

Used to indicate a shape, color, or size:

• This painting is mostly in blue.

Used to express while doing something:

• In preparing for the final report, we revised the tone three times.

Used to indicate a belief, opinion, interest, or feeling:

• I believe in the next life.

Information taken from: http://es.talkenglish.com/grammar/prepositions-on-at-in.aspx

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