Learn English – prefix for “infinite”

Depending on context quasi could work. Quasi as a prefix indicates similarity or resemblance. If a sphere is a ball and a hemisphere is half a ball, quasi-sphere would mean something that's ball-like, "roughly" or "more or less" a ball.

The "fore" in "foreshadowing" is a different prefix, etymologically speaking, from the "for" in "forgiven," and both are different from the "forth" in "forthright." Because they are etymologically distinct, each will have its own "opposite" prefix.

"Fore," as in "foresight" and "foreplay," means "prior," so some of its opposites are "hind-", "post-", and "after-".

The "for" in "forgiven," according to Oxford American, isn't actually a prefix; the word came down in one whole multisyllabic chunk from the Dutch "vergeven."

And the "forth" in "forthright" comes from the same root as the "for" in "forwards," so its opposite would likely be "back," as you mentioned, or "trans."