1. To talk about the position of something (with no movement), we use in.
- Where’s Susie? In the bedroom.
- My mother’s the woman in the chair by the window.
2. When we talk about a movement, we usually use into.
- She came into my room holding a paper.
- I walked out into the garden to think.
After some words, both are possible. (For example throw, jump, cut, push).
We prefer into when we think of the movement, and in when we think of the end of the movement-
the place where something will be.
Compare
- She threw her ring into the air.
- She threw her ring in(to) the river.
We use in after sit down, and very after put.
- He sat down in his favourite armchair (NOT He sat down into…)
- I put my hand in my pocket.