"For-" is a dead prefix

A column in the OxfordWords blog explains the difference between "fore-" and "for-" as prefixes:

...the spellings for- and fore- aren’t completely arbitrary: there’s a logical reason for using one or the other, based on their meanings.

Fore- (strictly speaking, it’s a combining form) comes from Old English and is related to the word before. Fore- is added to words to form words which have a group of meanings related to these main ideas:

• before or in advance of someone or something in time (so today’s weather forecast tells you what the future weather is likely to be; if you are foresighted, you’re able to think and plan ahead).
• in front or at the front of someone or something in terms of position (a forecourt is an area in front of a building; your forehead is at the front of your head).
• in front of someone or something in rank or order (a foreman was originally a person who literally led others from the front, developing into the later meaning of ‘a leader or supervisor of people’).

For- also derives from Old English, but the meanings of the words that it forms are very different to those beginning with fore-. You’ll find it at the start of words which have meanings conveying these ideas:

• banning someone or something (if you forbid something you refuse to allow it).
• giving up or doing without something (so to forfeit something is to give it up).
• failing to do something (when you forget something, you don’t remember something that you should have done).

So if the word you want has one of the above meanings and has nothing to do with being before in time, place, or order, then it should begin with for-. ..

No words have been formed from for- for many hundreds of years: in contrast to fore-, it’s a dead prefix. This means there’s only a small set of current English words that begin with for-: here are the most common ones:

forbear	forfeit	forgive	forlorn
forbearance	forget	forgiveness	forsake
forbid	forgetful	forgo	forswear

Additional details at the link, especially re the pairs of words "forebear"/"forbear" and "forego"/"forgo."