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.
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).
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