Prepositions of time
AT
A certain point in the past
Example: at half past nine.
For night
Example: at night
For weekend
Example: at the weekend.
IN
Months / seasons
Example: in August / in winter
Time of the day
Example: in the morning
Year
Example: in 2010
After a certain period of time
Example: in an hour
ON
Days of the week
Example: on Monday
FOR
Over a certain period of time ( past till now)
Example: for 2 years.
SINCE
From a certain point of time (past till now)
Example: Since 2000
AGO
A certain time in the past
Example: 3 years ago.
TO
Telling the time
Example: five to seven (6:55)
BY
In the sense of at the latest
Example: I will be back by 7 o'clock.