How to Use the Preposition 'In'
Use "in" with cities, regions, counties, states, and countries
I live in Portland which is a city in Oregon.
She works in Seattle which is in King County.
Use "in" with spaces that you can physically walk into, or place something into. These could be inside or buildings or outside as well.
in a room / in a building (indoors)
in a garden / in a park (outdoors)
Let's meet in the gym after class.
I'm going to see Tom in that building over there.
I enjoy walking in the garden at dusk.
She's out jogging with her friends in the park.
Use "in" with bodies of water:
in the water
in the sea / river / lake / pond / ocean
That duck is swimming in the water.
You can see the fish in the water.
Unfortunately, there's a lot of pollution in this sea.
How many fishing lines can you see in the river?
Use "in" with lines:
in a row / line / queue
There are so many people standing in that queue.
Please stand in a row and let me count you.
You'll have to stand in that line over there.
How to Use the Preposition 'At'
Use "at" with places in a town, city or other community:
at the bus-stop / movies / shopping mall / park / museum / etc.
I'll meet you at the bus stop.
I saw Peter at the movies last night.
I was at the shopping mall and decided I had to buy this sweater.
Let's see the exhibit at the museum.
Use "at" with places on a page:
at the top / bottom of the page
You'll find the page number at the top of the page.
Make sure to read the notes at the bottom of the page.
Use "at" with places within a room or large space:
at the back / front of the class / room / stadium
I think you'll find him at the front of the class.
They're seated at the back of the bus.
How to Use the Preposition 'On'
Use "on" with vertical or horizontal surfaces that you can lay something onto, or attach something to:
on the ceiling / wall / floor / table / etc.
I left the magazine on that table. Isn't that a beautiful painting on the wall?You have such lovely candles on the mantelpiece.
Use "on" with islands:
I stayed on Maui.
Have you seen the volcano on the smaller island?
Use "on" with directions:
on the left / on the right / straight on
Take the next turn on the left.
His house is on the right.
Drive straight on to the stop light.