Why v becomes f and z becomes s in Dutch

A Dutch word never ends in the letters v or z. When a form would end that way, the letter switches to its partner: v becomes f, and z becomes s. That is why the plural brieven (letters) has a singular brief, and huizen (houses) has a singular huis.

Write f and s when the sound sits at the very end of a word; write v and z when a vowel follows and the sound moves inside the word. The f/v and s/z are the same pair of sounds — Dutch just spells the end-of-word position with the plainer letter.

1. Look at a longer form of the word where a vowel follows the consonant — usually the plural or the full infinitive: brieven, huizen, leven.
2. There you see the real letter: v in brieven, z in huizen, v in leven.
3. When that letter lands at the end of a word (nothing after it), swap it: v → f, z → s. So brief, huis, and the verb stem leef.

The same switch turns up in several places, because Dutch is always adding or dropping an -en ending.

• Singular from plural: the plural -en form keeps v or z, the singular flips to f or s. rozen → roos (rose), neuzen → neus (nose).
• Verb stem from the infinitive: drop -en and flip the letter. leven → leef, geloven → geloof (to believe), blijven → blijf (to stay).
• Adjective without an ending: lieve (dear, before a noun) → lief on its own; halve → half.

This spelling rule explains a verb trap. The simple past adds -te or -de to the stem, and the 't kofschip trick says a stem ending in f or s takes -te. But verbs whose infinitive ends in -ven or -zen take -de instead: leven → leefde, reizen → reisde.

The stems leef and reis only end in f and s because of the switch above — the real letter is v and z (leven, reizen). Dutch follows the real letter, so these verbs behave as if they ended in a voiced letter and take -de. The same v/z underneath gives a past participle in -d: geleefd, gereisd.

Do not confuse this v/z → f/s switch with the final -d that stays written d. A word like hond (dog) sounds as if it ends in t, but you still write d, because the plural is honden. There the letter does not change on paper; with v and z it does. That separate rule is covered in final -d or -t spelling.