Prepositions link their objects to the other structures within the sentence. They are used to show placement, time, direction, cause, and location. They can be used alone or in two-or three-word combinations. Unfortunately, prepositions are often used idiomatically and often contradict rules. Therefore, their use must be memorized. There are, however, a few rules that will help you.
Pattern to remember
At is used for a specific time.
Incorrect: We will convene to 10:00 A.M.
Correct: We will convene at 10:00 A.M.
On is used for a specific day.
Incorrect: The deadline is at Friday
Correct: The deadline is on Friday
In is used for a specific city.
Incorrect: She was born at San Francisco.
Correct: She was born in San Francisco.
On is used for specific date.
Incorrect: The contract was signed in September 5.
Correct: The contract was signed on September 5.