For is a combination of para/ por while to translates as a.
For, however, cannot be used to mean "by" like por and cannot really mean "in order to" like para.
Quiero ir a la playa/ I want to go to the beach
A mi, me parece que.../ To me, it seems that
Para/por tres dolares/ For three dollars
Lo hago para/por el/ I do it for him