On is used to describe the position of an object in relation to another.
The book is on the table.
The page is in the book. The words are on the page.
The poster is on the wall.
She works on the second floor. (Do not use at)
The friends are on the beach = Describing position.
The friends are at the beach = Describing location.
You’ll find him standing on the corner
He lives on an island. (This only works for small islands, not for the U.K. etc.)
He lives in the U.K. (not on)
The button is on the left.
The option is on the menu
You can find it on a map
I’m working on something.
Please knock on the door before you enter!
They are on the bed = on top of the covers
They are in the bed = under the covers
He’s in the taxi = sitting inside
He’s on the taxi = standing on the roof (only works for cars)
They are on the train = inside the train (if you want to specify standing on the roof, you have to say it)
Please don’t be late, be on time