Although the term 'principal part' is usually applied to verbs, the same phenomenon can be found in some languages in nouns and other word types.
A number of verbs have fewer than four principal parts: deponent verbs, such as hortŏr – hortāri – hortātus sum, "to exhort", lack a perfect form, as do semi-deponent verbs, such as audeō – audēre – ausus sum, "to dare"; in both cases, passive forms are treated as active, so all perfect forms are covered by the perfect participle.
Verbs in Ancient Greek have six principal parts: present (I), future (II), aorist (III), perfect (IV), perfect middle (V) and aorist passive (VI), each listed in its first-person singular form: One principal part can sometimes be predicted from another, but not with any certainty.
There are three verbs (in addition to their derivatives) with an irregular third person singular form in the present tense.
Regular verbs require no memorizing of principal parts, since all forms can be deduced from the infinitive.
This can be solved by memorizing the infinitive with the third-person singular perfect tense, which some teachers recommend.
In some other classes of weak verbs without 'a' as the thematic vowel, the present indicative singular undergoes more changes, but they are still to a large extent predictable.
Icelandic strong verbs have the following principal parts: It is possible to make the present subjunctive mood (þótt ég finni, "though I find") from the first principal part (að finna, "to find").
The present singular indicative in this class also undergoes more changes (i-umlaut, dental suffix assimilation etc.
However, some scholars believe that the conjugation could be regularized by adding another principal part to vowel-alternating verbs, which shows the alternation.
However, by including the first person singular, present tense, indicative mood form (hiero) as a principal part, and noting that the diphthong appears only when that syllable is stressed, the conjugation of herir becomes completely predictable.
They include doubling a consonant, adding accent markers, adding the letter e, and converting letters for example y becoming i. Irregular verbs are markedly more complicated, requiring seven principal parts of which few can be easily derived from the infinitive.
In Scottish Gaelic there are two principal parts for the regular verb: the imperative and the verbal noun, for example pòg – pògadh 'to kiss'.
The ten irregular verbs can, with only two or three small aberrations (unexpected lenition), be deduced from four principal parts.