Exceptions
1. Verbs that end in –ieren do not have ge- in their participles: telefonieren → telefoniert
2. If the verb stem ends in -t or -d an additional e is inserted: arbeiten → gearbeitet
3. Similarly, if the verb stem ends in a consonant (except “l” or “n”) plus n or m, an additional e is inserted: öffnen → geöffnet