Solomon Golomb, A&S '51
Many nouns ending in –F (or –FE) replace this ending with –VES when adding –S as a plural (e.g., WOLF to WOLVES), or when forming the third person singular of the corresponding verb (e.g., STRIFE to STRIVES). How many examples of this can you find? (12 = good, 15 = better, 18 = excellent)
Several verbs ending in –VE have related words (usually nouns) ending in –FT (e.g., GIVE/GIFT). How many of these can you list? (5 = good, 7 = better, 10 = excellent)
^The trend toward "regularity" has made these plurals less common than simply adding –S.