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.
| infinitive
| english
| crude stem
| stem
| correction
|
| leven
| to live
| lev
| leef
| long vowel correction and replacing v by f
|
| lozen
| to dump
| loz
| loos
| long vowel correction and replacing z by s
|
| werven
| to acquire
| werv
| werf
| replacing v by f
|
| wuiven
| to wave
| wuiv
| wuif
| replacing v by f
|
| beven
| to tremble
| bev
| beef
| long vowel correction and replacing v by f
|
| durven
| to dare
| durv
| durf
| replacing v by f
|
| bonzen
| to pound
| bonz
| bons
| replacing z by s
|
Questions? 
