In summary we would like to point out the following: Verbs are devided in groups according to their ending and according to the behaviour in conjugation.
The verbs' endings are: -er, -ir, -oir, -re.
Of these groups, most of them are ending in -er, and within this group most verbs are regular, meaning that the stem is more or less unchanged when adding the conjugational ending.
Verbs ending in -re, -oir and some ending in -ir have the same conjugational endings.
Verbs ending on -ir have still two other patterns of endings, one identical with verbs ending in -er, one with the section -iss.
appeler (to call)
partir (to go)
finir (to finish)
il / elle
ils / elles
Having a look at only the endings, we see that changes are actually mainly in the singular conjugations, less in the plural conjugation.
Conjugational endings in an overview
ir (with iss)
ir (= oir, re)
oir (= ir, re)
re (= ir, oir)
This above is a chart that shows how it mainly works. To have a complete summary
of all the 2000 French verbs with their 80 patterns, it would be best to buy a simple verb table. There you will easily find the correct pattern for each verb. However, it is not to forget that mostly the changes are not in the pronunciation, actually they exist to keep the pronunciation. Therefore, speaking is then much easier than writing.
Variety of changes
1) Written irregularly, spoken regularly
For instance with verbs that end on -d or -t, the ending is omitted. That does not change the pronunciation, only has an impact on the writing.
2) Written and spoken irregularly
Here we have the real changes in the stem and the minor changes of vowels, as we have seen before.