What is the verb form of the noun friend?
I know it is to befriend, but I am confused as to how to teach this to my daughter.
When is the prefix be used to make verbs? Is "befriend" an exception?
Befriend is an actual verb that is linked in meaning to the noun friend.
There are more such pairs, and sometimes one can be formed from the other. There may be some patterns, but I would indeed not go looking for rules. Semantically linked pairs or triplets of nouns, verbs and adjectives have been used throughout history, and they have been borrowed from one language to another, sometimes as a set, sometimes individually.
A favourite example of mine is that an undertaking is not linked semantically to an undertaker in English. Since that word was used already for someone who buries people, the French entrepreneur was borrowed. Entrepreneurship was then formed on English rules, and borrowed back into French.
If no verb exists yet for a noun, people sometimes use the noun unchanged as a verb. In some cases, those “verbed nouns” actually catch on, but usually they are a one-of. But someone who adds a point to a meeting agenda can be said to table an issue. The (proper) noun Google is now also used as a verb.
It can be very interesting to try and find patterns in word-formation, but rogermue mentions in their comment, beware of defining “rules” for it. Generations of language users have shown an incredible creativity in forming words, and just about any “rule” you find will likely have more exceptions than words that follow it.
The noun friend is actually a nice example, because the verb befriend has been around for some time, but when (FaceBook, wasn't it) was looking for a verb with the opposite meaning, they coined unfriend, which looks like it is based on a verb friend.