What's the difference?
Why do I say get in the car but get on the bus?
Think about this one. When you get in a car (or getting into a car), you then sit down. But when you get on a vehicle or craft that, after you enter, you walk inside to then find your seat (if there is one available), then you are getting on it. So you can get on a bus, a plane, a boat, a train – any vehicle where you are able to walk around inside the space that is available.
Now any transport that requires you to sit on a saddle like a bike – requires you to get on it, that is, to physically swing one leg over to sit down and then place your bottom on the saddle, and not to go through a door – then you get on it. You get on a bike, a quad, a motorcycle, a scooter, and so on.
Fall off and fall from, for example, 'to fall off a cliff' and 'to fall from a cliff'?
This depends on the situation and the context. Fall off and fall from can be used in several ways (e.g. shares fell from 0.3% to 0.25%, By the fifth year, children's progress at school often falls off because they get bored), so we will focus on the situation given above.
If you fall off a cliff, this focuses on the fact that you were on the cliff, and that for some reason, you had an accident. Gravity did its business, and because you had nothing to stop you, you landed on a lower place. Probably the ground.
- How did you break your leg?
- I fell off a cliff - there was no fence, it was dark, and I didn't see.
Unlike fall off, fall from is not a phrasal verb, and so if someone fell from a cliff, then this just simply tells us that they fell, and it just happened to be from a cliff. This does not tell us how or why, and is often used in the third person. Compare:
He fell off a cliff. He slipped, and there was no fence.
He fell from a cliff. As to why that happened, I don't know. Maybe it was an accident.