In denotes a state of rest or being inside something. Into indicates movement to the inside of something.

• He is in his bed.
• He is in town.
• He fell into the river / pond / sea / pit.
• He walked into the room.

On and upon
On is generally used to talk about things at rest. Upon is used to talk
about things in motion.

• He sat on a chair.
• He jumped upon his horse.

Note
This rule is no longer strictly followed, and on is often used to talk about
things in motion too.

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