Most nouns in English have a very regular plural form in which /s/ or /es/ is added to the singular form (car/cars, pass/passes). Some nouns have a more irregular form in which other changes occur between the singular and plural forms. These words have preserved older forms from Old English and Middle English.

In one such change many words that end in /f/ or /fe/ change the /f/ to a /v/ before adding /s/. These words include:




Explanation

These forms are irregular and as such simply have to be memorized.

History

The reason why these words change form comes from the history of the language. The Old English alphabet had fewer letters than the current 26 of Modern English.

Remember that Old English used endings for most nouns. These endings changed form depending on case, gender, and number.

The consonant sounds of any language can be either voiced or unvoiced. In most languages these sounds are represented by different letters in the alphabet which differentiate the sounds into those without voice and those with voice. In Modern English these are (t / d), (k / g), (p / b), (f / v), (s / z), (ch / dz), and (th / as in three / th / as in this).

English had a rule that said a consonant between two voiced sounds (vowels or voiced consonants) is always voiced. Old English did not have seperate letters for all of these sounds. So, the sound simply changed from voiceless to voiced without changing the letter itself. In Middle English and Modern English there are seperate letters for the voiced and voiceless equivalents of these sounds, so those are now used. The only exception to this is the voiced and voiceless /th/ which has the same spelling in voiced or voiceless form, and sometimes the /s/ which sometimes changes to /z/ but sometimes remains an /s/.

So the reason these words change from /f/ to /v/ is to represent the added voicing of that final consonant.[/img]