When, after subtraction of -en from the infinitive (and possibly the correction for length of the vowel), the stem ends in v or z, these letters turn into f and s respectively.
A stem never ends in v or z
See also [voiced and unvoiced consonants](#/lesson/SpellingAndPronunciation.23).
infinitive | english | crude stem | stem | correction
leven | to live | le v | le e f | long vowel correction and replacing v by f
lozen | to dump | lo z | lo o s | long vowel correction and replacing z by s
werven | to acquire | wer v | werf | replacing v by f
wuiven | to wave | wui v | wuif | replacing v by f
beven | to tremble | be v | be e f | long vowel correction and replacing v by f
durven | to dare | dur v | durf | replacing v by f
bonzen | to pound | bon z | bons | replacing z by s