In: Can be used for months, years, decades, centuries, seasons, and the future.
1.) I will go to university in September.
2.) I went to university in 1993.
3.) I went to university in the 1980’s.
4.) I went to university in the late 20th century.
5.) I go to university in the fall.
6.) I will go to the store in a few minutes. Note that this use of “in” is in the future, but it is for a shorter period than the other examples.
At: Can be used for particular times.
1.) I will go to the store at 5:30.
2.) I go to the store every day at midday.
3.) I go to the store on Tuesdays at lunchtime.
4.) I go to the store at sunset.
5.) I go to the store at night. Note that we do not say “at day.” Instead, we say, “during the day.”
6.) I go to the store at Thanksgiving to buy enough food for the meal.
7.) British English usage: I go to the store at the weekend. American English would say, “on the weekend.”
On: Can be used for days and dates
1.) I like to go to parties on Fridays.
2.) I am excited to go to the party on January 19.
Check out these sites for some online practice! Then try making your own sentences using these prepositions.