The canonical form of a verb used to form the present perfect, past perfect, and future perfect tenses. Those three tenses are distinguished by the form of the helper verb used. The helper verb used for the present perfect tense
For most English verbs, called "regular verbs", the form has verbed forms the verb's past participle. has talked, has requested, has played.

The rest, irregular verbs, take on past participles that do not follow any known rules. However, several forms are common: The irregular forms are probably survivors of the various languages that had to accomodate each other (Anglo-Saxon, Norse, French) to form the English language. However, some of these irregular forms are mirrored precisely in German, so more ancient accomodations may have some influence.
German has both regular and irregular verbs, but its rules are a bit more complicated than in English. For one thing, each verb uses one of two different helper verbs, haben or sein. For the present perfect, the third person singular form (ist or hat) is canonical. For the past perfect, hatte or war. For the future perfect, the appropriate form of werden (wird) is used instead. Most "sein" verbs indicate a change of position or condition, or are intransitive.

The past participle of a regular verb ends in -t or -et (hat gesagt, hat geregnet). The past participle of an irregular verb usually contains a vowel shift and usually (but not always) ends in -en (ist gefunden, ist gewesen, hat gegeben).

There are specialized rules for the prefix a German verb gets for the past participle:
  • The prefix is usually ge-
  • .
  • If the verb begins with an inseparable prefix, there is no prefix. (ist verschwunden, ist verloren, hat bedeutet).
  • If the verb ends in -ieren, there is no prefix (ist studieren),
  • If the verb begins with a seperable prefix, the prefix, as you might guess, is separated from the verb, and stuck onto the past participle of the root verb (ist abgefallen).
German also uses a verb's past participle to form the passive voice, along with the corect form of "werden" (wird gebracht). The past participle of a passive voice form is ist (pp. pf root verb) worden. "Worden" is used only in this context, it is a hallmark of a passive voice construction.

The past participle of a verb with a modal auxilliary results in that peculiar construction, the double infinitive, which requires its own node.