Why do some verbs change in only some persons in the present tense?

There are many 'radically changing' verbs which change their root in the present tense in only some persons of the verb. These are always in all persons EXCEPT nous and vous. For example the verb appeler gains an extra L: J'appelle               Nous appelons Tu appelles            Vous appelez Il appelle              Ils appellent There are often patterns to spotting these verbs for exampler -ELER verbs follow this pattern. Be careful though because this isn't always the case!