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f and v are partial allophones (two phonemes that aren't always recognized as such, at least graphically). Usually we change the letter when the sound changes (leaf/leaves, sieve/sift, weave/weft), but not always (dwarfs), and /f/ once represented both phonemes in English.. The difference is entirely whether it is voiced (v) or unvoiced (f), which is entirely situational. for some phonemes we still don't distinguish in normal use (e.g. /th/ for both voiced and unvoiced phonemes), and many others we don't change the spelling when there are voiced/unvoiced phonemes in different derivatives of the same word or suffix. e.g. the /d/ in -ed is not changed to t when unvoiced (except in some older forms like slept, wept where the preceding vowel is shortened or unvoiced as well).