Currently in my logic class we are learning about quantifiers, and I have a question that our book doesn't fully explain.

It says that if you have a formula like:

for all x : (f x)

then you can prove it by simply proving:

(f x)

but it then says that they are not propositional equivalent, and I do not understand why they are not propositional equivalent. If someone could explain why that would be great.

Thanks