Prepositions. These little guys can be very confusing. In my opinion, the easiest way to learn prepositions is to associate them with a new word or phrase. I’m going to assume you know most of the words that are commonly used with prepositions though, so I’ll do my best to explain the difference in use with examples.
The first important thing to remember is that we use prepositions most commonly with words or phrases about place and about time. I’ll deal with place first, then move on to time.
On, in or at (place)
On refers to
any position on a surface: your dinner is on the table.
a position along a road or river, or next to a body of water like a sea, ocean or lake: I live on Baker Street/we have a holiday home on Loch Ness.
a floor in a building: we have a lovely apartment on the 21st floor.
Being on a particular form of public transport: I’m on the bus but it’s running late. NB if you use a mode of transport regularly, use by: I travel by train every day.
In refers to
talk about a position in an enclosed space: I’ll be in my room if you need me/She’s in the car.
talk about a position in a large open space: they are hiking in the countryside for the day.
class (a lesson in school or university: my daughter finds it difficult to concentrate in class
At refers to
refer to a position location which we see as important: I’ll be at my desk all afternoon.
Talk about location companies and organizations: how long have you been working at Google?
Activities involving a group of people: I think we met at Jim’s party.
School, college and university: My son is at university.
Most shops and places where we receive treatment. I have an appointment at the doctor’s on Friday/I’m at the grocers’, do you need anything?
On, in or at (time)
On refers to
Dates: I start my new job on the 1st of October.
An occasion on a single day of the week: Jane’s birthday is on Tuesday.
A repeated occasion on plural days of the week: We don’t have class on Mondays.
In refers to
Parts of the day: See you in the morning!
Months: we usually go on holiday in August.
Years: this house was built in 1955.
Seasons: I love to see the colours in autumn.
Long periods of time: many people emigrated to America in the 19th century.
At refers to
A particular time: I’ll meet you at the restaurant at 8pm.
A particular point in the day: I was still awake at midnight.
Particular points of the week: the weather was so bad at the weekend.
Special celebrations: will you be home at Christmas?
OK. These are the most common uses. Once you are happy with these you can find more subtle differences at the bottom of this webpage: https://dictionary.cambridge.org/grammar/british-grammar/at-on-and-in-time