'to fall in with' means to become friendly with a group.
- He stopped going to school when he fell in with a bad crowd.
- At university, she fell in with a group that introduced her to the theatre.
'to fall into' a category or group means that it belongs to that group.
- It isn't a romantic comedy or a drama. It doesn't really fall into either category.
- The work falls into three distinct parts; administrative, planning and financial.
'to fall off' means to separate from something it was attached to.
- When I got home I was surprised to see that picture had fallen off the wall.
- I still haven't picked up the apples that fell off during the strong winds last week.
'to fall off' also means to become less or lower.
- Demand for our products is falling off in Europe although it remains high in America.
- The number of tourists falls off during the wet season but quickly goes back up as soon as it is over.
'to fall out' means to have an argument and no longer be friendly with that person.
- It's not that important. I don't want to fall out over it.
- They fell out over money when they started a business together and haven't spoken since.
'to fall out' also means something drops to the ground from the container it was in.
- When I opened the cupboard, it fell out and broke on the floor.
- I held on very tightly to the side of the boat. I couldn't swim so I didn't want to fall out.
'to fall over' means to become unbalanced and end up lying on the ground.
- I tripped on the edge of the sidewalk and fell over into the road.
- During the play, some of the stage set fell over and hit one of the actors on the head.
'to fall through' is used about a plan or arrangement that goes wrong and cannot be completed.
- I'm not doing anything this weekend. Our plans have fallen through.
- The deal fell through when the seller started demanding too much money.