past participle(as adjectives)

[Allthewayanime]

I would like to know when the past participle can't be used(as adjectives).For instance:

e.g. She's a very straight-forwarded girl.
(I know that's incorrect but I want to know why)

[Dozy]

There is no such word as "straight-forwarded". For that to exist as a past participle, there would have to be a verb "to straight-forward", which there isn't. There is a verb "to forward", and you could talk of "a forwarded letter", for example. However, the "forward" part of the adjective "straightforward" is not a verb.

(Please note spacing errors again. In your first line you need a space before the opening bracket and after the full stop.)