In Keller's Learn to Read Latin:
In the third principal part of capio, capere, ce(long)pi, caphls, the root vowel changes to a long -e-. The change of the vowel indicates a change in tense. A change in root vowel that corresponds to a change in meaning is called ablaut. This phenomenon is common to all PIE languages. Cf. English sing, sang, sung; song
do, dare, dedi, dams is an irregular first-conjugation verb because its principal parts do not follow the pattern of other first-conjugation verbs, and its present stem, da-, contains a short -a-. However, two forms have -a(long)-: the second person singular present active indicative (dis) and the second person singular present ac- tive imperative (di). (For the indicative see �9; for the imperative see �32.) MEMORIZE THESE FORMS. The third principal part, dedi, is called reduplicated because the first consonant of the root, d-, has been doubled with an intervening vowel. like all verbs of giving, showing, and telling, do regularly takes both a direct object and an indirect object.
Are ablaut and reduplicated phenomena two exceptions that occur during deriving the perfect active stem (obtainable from the third principal part (indicative, active, perfect, singular, first-person form)) from the present stem (obtainable from the second principal part (indicative, active, present, singular, first-person form))?
What are the regular rules that govern the derivation of the perfect active stem from the present stem? Or the deriviation in the other way around?
What are the regular rules that govern the derivation of "the perfect passive stem" (the name I gave to the stem of the fourth principal part, when it is a form of the perfect passive participle) from the present stem or the perfect active stem?
Thanks.