in

in is on the Basic English 850 List.

impulse - in - increase

in is one of the 1000 most common headwords.

improve - in - include

Preposition

It is in the box

Used to show that something is inside something else.
The cat is in the box.
Used to show that someone is at home, or is available.
Is John in?

The Doctor is now in.
Used to show movement towards the inside.
The rain came in through the window.
Used to introduce an explanation or clarification of the previous statement with a that clause.
Toronto is a little different, in that it's not quite as cold.