We use IN to show position, whereas we use INTO for movements toward the inside of something; e.g.

COMPARE:
a) She was in the house. (static position)
b) She went into the house. (movement)

1-a) They are sitting in the bus.
b) They climbed into the bus.

2-a) The pencils are in the box.
b) I’m going to put these pencils into the box

i) She was walking in the garden. Then she walked into the house.
ii) Stop running around and get into bed.
iii) I can’t get into these trousers any more. They’re far too small for me.
iv) They climbed into the truck and drove away.
v) The door opened and a nurse came into the room.
vi) Crack three eggs into a bowl and mix them together.
vii) He thrust his hands into his pockets.

For more chapters/topics on English Grammar read the following book authored by me.

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For English Practice Sets on various topics read the following book authored by me.

Link for buying the above book