Prepositions don't have to be part of a phrasal verb, in fact I would say most aren't. In this case, none of the prepositions have any special relation to the verb. They're all literal descriptors of the positions the subject goes through during the action.
Try to break up the sentence into the smallest chunks that make sense. Here: He fell. Yeah, that makes sense, that's true. Okay, where did he fall? Into the pool. What did he fall off of? He fell off the ledge. So now, put it together.
He fell off the ledge and into the pool.
You can leave out the "and" here, but it's still there for grammatical purposes. You can tell if you use three prepositions:
He fell off the ledge, through the air, and into the pool.
You can't leave out the "and" here, there's no way to use three prepositions without the conjunction. That means that there's no way to do it with two, either, and the "and" is still there, it's just not spoken.
Right and straight are both intensifiers, and don't really make sense to me here. There's not exactly an indirect way to fall off a ledge and into a pool.
I hope this helps.
Edit: A note to help distinguish between phrasal verbs and normal prepositional adverb phrases: phrasal verbs don't accept objects of the preposition, and they don't pair with prepositions in conjunction phrases.
The roof fell in. good
The roof fell in the house. not good
The roof fell in and onto the ground. not good
The roof fell in onto the ground. good
Does that help you see the difference a little better?
Best Answer
"She bit into an apple." This is grammatical and makes sense on its own. In the story of Snow White, biting into a poisoned apple is an important part of the story.
"She bit off an apple." doesn't make sense without more context. Usually we would say something like "She bit off a piece of apple." To "bite off", a piece needs to be separated from the whole with your teeth. If there was an edible apple tree like the one below, she could bite off an apple (from that tree).
There is an idiom that might help explain it - to bite off more than you can chew. Taken literally, it means that the piece you bit off is so big, it fills your whole mouth making it hard to close your jaw to chew it. I picture something like this
Some more examples:
The dog bit into the man's leg. (Doesn't tell you if the dog has let go, or if the dog has taken a chunk out of the leg.)
The dog bit off the man's nose. (Most or all of the man's nose was separated from his face. Ouch!)
The dog bit the man. (Doesn't tell you where the dog bit the man, or whether it bit anything off, or how severe the bite was.)