From Wikibooks, the open-content textbooks collection

Principal Parts

All Latin verbs are identified by four principal parts. By using the four principal parts, one can obtain any and all forms of the verb, including participles, hi gerunds and the like.

Examples of principal parts from verbs of each conjugation:

1st: ambulō, ambulāre, ambulāvī, ambulātum (to walk)

2nd: doceō, docēre, docuī, doctum (to teach)

3rd: mittō, mittere, mīsī, mīssum (to send)

4th: audiō, audīre, audīvī, audītum (to hear)

For all regular verbs, the principal parts consist of the first person singular present active indicative, the infinitive, the first person singular perfect active indicative, and the supine (or in some texts, the perfect passive participle).

• Deponent verbs have only three principal parts:

patior, patī, passus sum (to suffer)

ūtor, ūtī, ūsus sum (to use)

• Likewise, semi-deponent verbs have only three:

audeō, audēre, ausus sum (to dare)

gaudeō, gaudēre, gavīsus sum (to rejoice)

Some verbs lack fourth principal parts (e.g., timeō, timēre, timuī, —; to be afraid); others, less commonly, lack a third in addition (e.g., feriō, ferīre, —, —; to beat). Others, such as sum, esse, fuī, futūrus, may use the future active participle (futūrus) as their fourth principal part; this indicates that the verb cannot be made passive.