I have a single belief, but i believe in many things

I have a single knife, but at set of knives

The bud had a single leaf, but a tree has many leaves..

The general rule is f changes to ves as words become plural..

But what about motif as a theme-- a recurring salient thematic element, a floral motif would includes many floral elements.

But if I have a collections stamps (for stamp art) -- I have a set of florals, and a set of coffee related stamps, and a set of trees. Do i have several motifs? of several motives?

This web pages says motives-- Take a glance at the available motives.
http:// http://www.magical-stamping-tour.com/seite_4/seite_4.html

M-W10 says motives are something (as a need or desire) that causes a person to act. , 2) a recurrent phrase of figure that is developed throught the course of a musical composition., and finally :MOTIF

So which is right? and are there words where what appears to be the plural is a totally different word?

well, I would use motif/motives for musical purposes, as singular/plural.
the plural version has different uses, and the singular is really the oddball, though I suppose the repeating pattern(as in stamping or wallpaper) supplies the idea of motion.

The general rule is f changes to ves as words become plural

The general rule stems from the linguistic fact that, in Old English, the f (along with a couple of other unvoiced consonants) became voiced when between two consonants. Thus OE wif, pronounced [weef] and meaning woman was wifes, pronounced [wee-ves] in the plural. Thus Modern English wife, wives. This rule can slop over onto words brought into the language when the force of the rule is still what the linguists call productive.