How to use IN ON AT in expressions of time

These 3 pictures are designed to sum up prepositions of time in a way that is easy to remember.

BIG PERIODS – IN

In the first picture the box is so big that the stick man can get IN it.

This box is for big periods of time:

• “In the 4th century the Roman empire declined”
• “In 1979 Mrs Thatcher became Prime Minister”
• “In January we had snow”

MEDIUM PERIODS – ON

In the second picture the box is quite big – it is not big enough for the stick man to get IN it, but he can stand ON it.

This box is for medium sized periods of time:

• “On Thursday I shall go home”
• “On Wednesdays we go shopping”
• “On 26th July it is my birthday”

SHORT PERIODS – AT

In the third picture the box is too small for the stick man to get IN it or ON it – but he can stand AT or beside it.

This box represents short periods of time:

• “At 2pm the lesson will end”
• “At 12:23pm my baby was born”
• “At the 3rd stroke it will be 3:34 precisely”

I hope that helps you to remember which preposition of time to use. 

Remember that there are many exceptions, and many other rules we haven’t covered here, but this 3 boxes will help you construct many correct sentences.