The uses of "in" and "on"

Question

A reader wrote, “When do you use in vs. on?  I always have trouble with their use.”

Answer

Everyone has trouble learning how to use the prepositions in and on! There are basic guidelines to follow, but there are also many uses of these two words that can’t be explained with the basic guidelines – you just have to memorize them. 

Basic guidelines for in and on
Here are the basic guidelines:

In general, in is used to indicate location or position within or inside something:

• We went for a swim in the lake.
• They have a house in the country.
• Albuquerque is in New Mexico.

In general, on is used to indicate touching and being supported by the top surface of (something), or moving to a position that is supported by something:

• The book is lying on the table.
• There is a lot of frosting on the cake.
• I climbed out on the roof.

Other uses
In is also used to indicate:
1. That someone or something belongs to or is included as part of something

• She used to play in a band.

2. During a period of time, a season, etc.

• It happened in the 1930s.

3. At the end of (a period of time)

• I'll be there in a minute.

4. How people or things are arranged

• They stood in a circle.

And on is also used to indicate:
1. The part or object by which someone or something is supported

• How long can you stand on one foot?

2. That something is attached to something

• He hung the painting on the wall.

3. Where someone or something is hit or touched

• I bumped my head on a low branch.

4. The surface or part where something is seen or located

• There are marks on the wall.

And there are many other uses for both of these prepositions. To see a full list, look at the entries for the prepositions in and on in Merriam-Webster's Learner’s Dictionary online.