a) weak verbs with suffix -t     (no change of vowel) b) strong verbs with suffix -en     (change of vowel) c) strong verbs with suffix -t     (change of vowel) d) strong verbs with suffix -en     (no change of vowel) warten => gewartet (to wait => waited) bringen => gebracht (to bring => brought) sehen => gesehen (to see => seen) gehen => gegangen (to go => gone)

In addition to these four different types there are those ones that are totally irregular. The irregular ones do not have a prefix ge- and the endings are more or less unpredictable. There are only a limited amount of verbs that are so irregular.

I) verbs that end on -ieren => ending -t, no prefix (e.g. studieren [to study] => studiert)
II) verbs that have already a prefix do not use ge-; that can be ver-, emp-, zer-, be-, er-, vor-
the next hints are more a hint than a rule. Basically you have to learn them by heart, but it might help to keep in mind that
IIa) verbs with prefix ver-, emp- mostly have a perfect participle suffix -en
IIb) verbs with prefix be-, er-, vor-, zer- mostly have a perfect participle suffix -t.

kapieren => kapiert ramponieren => ramponiert verstehen => verstanden empfinden => empfunden beenden => beendet vorbereiten => vorbereitet berichten => berichtet versuchen => versucht erobern => erobert erreichen => erreicht beschließen => beschlossen zerstören => zerstört vergessen => vergessen beachten => beachtet verursachen => verursacht erweitern => erweitert errichten => errichtet to understand => understood to break => broken to understand => understood to feel => felt to finish => finished to prepare => prepared to tell => told to try => tried to conquer => conquered to achieve => achieved to decide => decided to destroy => destroyed to forget => forgotten to consider => considered to cause => caused to extend => extended to erect => erected