Phonetics
A simple rule to remember the spelling of certain conjugations is to know a few rules of phonetics of the language. Indeed, some letters change or appear in order to keep the sound of the verb in other forms.
Sound gue and ge
All verbs in -guer, like for instance fatiguer (to tire), keep their u in all conjugations. As for -ger verbs, it is sometimes necessary to add an "e" in front of the standard ending in order to keep the sound "j" only in front of the vowels a, o and u. The sound "j" is not modified in front of the "e" and the "i".
- je mange, je mangeais - Elles se blasent ainsi souvent d'elles-mêmes en fatiguant leurs désirs dans le vide" (BALZAC, Illus. perdues, 1843).
Sound se and ze
We get the sound "se" when the syllable beginning with the letter "s" is at the beginning of a word or follows another syllable ending with a consonant.
- servir, danser
The sound "se" is obtained by doubling the s between two vowels (s alone gives ze)..
- positiver, posséder
Exceptions mainly concern words created from prefixes such as de- or re-.
- resaler, resurgir, désulfurer