1. A phrasal verb consists of a verb and one or more prepositions or an adverb which modifies the meaning of the verb.
"walk down" is not a phrasal verb because "down" does not change the meaning of the verb "walk". It shows that the action is moving from a higher position to a lower one. This is the literal meaning of down and is found in many different combinations.
e.g. I fell while I was walking down the stairs/the street..
"A phrasal verb consists of a verb and a preposition or adverb that modifies or changes the meaning; 'give up' is a phrasal verb that means 'stop doing' something, which is very different from 'give'. The word or words that modify a verb in this manner can also go under the name particle".
2. Prices have fallen off lately[ have fallen off = have decreased]
She fell off her bike [off -shows the movement downward]
Off functions as a preposition of position or movement and is the converse of on. We speak of getting on a bus and off a bus, taking things off the table ...
He fell off the cliff.
In the above sentence, the word off functions as a preposition.
The Adverb "off"
1. In a direction away from the speaker or object.
He drove off in a cloud of smoke.
The book fell off the table.
2. Into a state of non-operation; into a state of non-existence.
Please switch off the light when you leave.
Used in many phrasal verbs, off is an adverbial particle often mistakenly thought of as a preposition. (It can be used as a preposition, but such usage is still rare and always informal;