I can't know the difference between "fall" and "fall down", I saw both definitions in Cambridge and in some dictionaries, but they seem to be the same to me.
See these definitions:
Cambridge Fall – to suddenly go down onto the ground or towards the ground without intending to or by accident.
Cambridge Fall down – to fall to the ground
Cambridge Fall vs Fall down – We can use fall as a noun or a verb. It means ‘suddenly go down onto the ground or towards the ground unintentionally or accidentally’. It can also mean ‘come down from a higher position’. As a verb, it is irregular. Its past form is fell and its -ed form is fallen. Fall does not need an object.
*We can’t use fall down to mean ‘come down from a higher position’:
House prices have fallen a lot this year.
Not: House prices have fallen down a lot …*
What does it mean? Why is it not allowed to use fall down when something comes down from a higher position? If something falls down, according to the Physical laws, the object had to be in a higher position, if not, how did it feel "down"? It doesn't make any sense to me, how can something fall down without being in a high position?
Can anyone explain to me the difference between these two terms, because I read it many times but I still don't get it, as far as I'm concerned, it may be optional.
Best Answer
English has a lot of verb+transitive preposition constructions like this one—hurry up, turn around, deal out, gather in—in which the preposition doesn't add any particular directional or locative meaning, even a figurative one.
When a construction of this sort first emerges you often find pedants and purists complaining that the preposition is "superfluous" or "redundant" (for instance, Google a fairly recent one, "continue on"), and it's true that the core meaning of fall down and hurry up and the rest are expressed perfectly adequately by the bare verb. But this misses the point: the function of the preposition is not so much to modify the meaning as to intensify or 'color' it.
If you are a speaker of German or a Romance tongue it may help you to think of the preposition as equivalent to the prefixes on verbs which are common in your language. (English used to create verbs with prefixed native prepositions this way, too, but mostly gave it up about eight or nine hundred years ago in favor of posterior prepositions.)
What I think Cambridge is trying (not very successfully) to express is that fall down is not used to describe mere movement from a higher position to a lower one: it typically characterizes the movement as sudden, rapid, involuntary, uncontrolled, or some combination of these; and it usually implies that the fall ends in a more-or-less violent collision with an unyielding surface. When prices fall they usually stop somewhere short of $0.00; but when a person falls down she usually lands on the ground or the floor with a bump!—she falls all the way down.