+0
the plural for leaf is leaves
the plural for beef is beeves (?) 
the plural for deaf is deaf
the plural for motif is motifs
what's the rule ?? 
Comments  
I'm not aware that beef and deaf have a plural form, actually.

Without a good rule to fall back on, use the dictionary. It will list the plural form if it is anything other than just adding an S.
My dictionary says that beef is uncountable when you mean the meat. So, there is no plural. However, there are two plural forms for "beef" for other meanings.
1. Beef - an adult cow raised for its meat. The plural is beeves.
2. Beef - (slang) a complaint, an argument or dispute. The plural is beefs.
However, I guess the above usage is rare.

When used as a noun, "the deaf" is used collectively and takes a plural verb. That doesn't mean "deaf" is the plural for "deaf". You can't use "deaf" as a noun in the singular or as a countable noun. "One deaf", "two deaf" are wrong.

I don't think there is a general rule for nouns ending with "f" except that "-ff" becomes "-ffs".
Teachers: We supply a list of EFL job vacancies
whoa beeves? i've never heard that. Emotion: wink

i don't really think there's a rule of "f"s. just follow your instinct. when in doubt, consult your friend, the dictionary. 
the rule is there must be a vowel before the "f" in order for the plural to end in -ves. For instance, elf=elves, wolf=wolves, scarf=scarves, staff=staves
Anonymous... must be a vowel before the "f" in order for the plural to end in -ves. For instance, elf=elves, wolf=wolves, scarf=scarves, staff=staves
Three out of four of your examples do not have a vowel before the "f", so what exactly does this prove? Why have you chosen examples that don't even obey the "rule"? Did you just make up this "rule"? Emotion: smile

CJ
Students: We have free audio pronunciation exercises.
My reference (CGEL) has no rule, just a list of

The irregulars: calf, elf, half, knife, leaf, life, loaf, self, sheaf, shelf, thief, wife and wolf. Some - scarf, at least - are obviously missing, though.

Both plural forms (-fs and -ves): dwarf, hoof, scarf, wharf.

Other nouns ending in -f(e) have regular plural -f(e)s, it says.

Exceptions mentioned are still lifes and the Toronto Maple Leafs. No mention of beef. Where's the beef ?!