The element for-, though it stems from the same Proto-Germanic word that gave us the preposition for, deviated from the common ancestor to serve as a prefix meaning “away,” “opposite,” or “completely.” That’s the sense that contributes to the meaning of most words beginning with for-.
Notice that these words have in common that their connotations are definitive: The verbs forbid (“prohibit,” with a root cognate with bid and meaning “command”), forget (“fail to remember” or “inadvertently neglect,” with a root cognate with get and meaning “grasp,”), and forgive (“pardon,” with a self-evident root) are potent; so, too, is the adverb forever (“always,” with a self-evident root).
Other words in the for- family have not only that strong sense but also an archaic ring to them, and most are rare: the verbs forbear (“refrain” or “endure”; the root is self-evident), forfend (“prevent” or “protect”; the root, fend, is the word meaning “ward off”), forgo (“refrain from”; the root is self-evident); forlorn (originally “disgraced,” later “wretched” and “abandoned”; the root—also seen in lovelorn, meaning “pining for love”—means “lost”), forsake (“abandon”; the root, cognate with sake, means “blame” or “dispute”), and forswear (“renounce”; the root is self-evident) and the adverb forsooth (“indeed”; the root, cognate with sooth, means “truth”).
Other intensive terms, which are so archaic as to be obsolete, are the verb fordo (“destroy” or “kill”; the adjectival form, fordone, survives in an inverted version as the idiomatic phrase “done for”) and the adjective forblak (“exceedingly black”).
Several for- words are only partially related: The first syllable in the adjective foreign (“from beyond one’s own country”) and in forfeit, both a verb and a noun (“give up” or “something given up,” respectively), is from a Latin element meaning “outside,” which is distantly akin to the other for-.
The verb and adverb forward (the word retains a noun function only in reference to a position held by certain athletes) was spelled forewearde in Old English. Like words that still begin with fore-, it pertains to something located before something else (or, in the case of some of these words, something occurring before something else); for- and fore- are distant relatives. Fortune and fortuitous are unrelated outliers based on the Latin root that means “chance” or “luck.”
The second element in therefor and therefore—the distinction between the first rare form and the more common second one is “by reason of that” versus “in consequence of that,” respectively—is the conjunction for (“because”).