The Four Principal Parts
Every Latin verb advertises its full grammatical address through four principal parts — amō, amāre, amāvī, amātum. Memorize those four words and you have memorized the verb: every tense, every mood, every voice can be generated from them.
The four parts exist because Latin builds its forms from three different stems, and you cannot predict one stem from another.
Part 1 (amō) gives the present stem; part 2 (amāre) confirms which conjugation you are in; part 3 (amāvī) gives the perfect stem; part 4 (amātum) gives the supine stem, which builds the perfect passive and future active participles.
The trap is the perfect: moneō gives monuī (no -v-), tegō gives tēxī, capiō gives cēpī, currō gives cucurrī. Drill the four parts as a unit — there is no shortcut.
Four words, three stems. Every tense, mood, and voice in the verb is built from one of these three stems.
Part 2 alone tells you the conjugation; parts 3 and 4 are unpredictable and must be memorized verb by verb.
See It In Action
— Verg. Aen. i. 1
Canō is part 1 — present stem at work. The perfect cecinī (part 3) shows reduplication; you would never guess that from the present. This is why the dictionary gives all four.
— Verg. Aen. i. 12
Fuit is built on the perfect stem fu- — no visible link to the present sum. The fourth slot uses the future participle (futūrus) because sum has no supine.
— B. G. i. 46. 2
Both perfects use vowel lengthening: fac- → fēc-, recip- → recēp-. Same conjugation (3rd-iō), same trick — but you only know it because parts 3 and 4 told you. The supine factum resurfaces as the participle factus, -a, -um.
Students reflexively expect the perfect to be "present stem + -vī". That works for 1st and 4th — and almost no one else. Part 3 of the dictionary entry is doing essential work.
perfect = present stem + -vī across the board
moneō → †monēvī; tegō → †tegī; capiō → †capīvī
regularized — but wrong
each conjugation has a default but verbs break the rule constantly
moneō → monuī (u-perfect); tegō → tēxī (s-perfect); capiō → cēpī (lengthened vowel)
irregular by default — memorize part 3
Tip: Quick rule of thumb by conjugation: 1st verbs = mostly -āvī (regular). 2nd = mostly -uī with no -v-. 3rd = grab bag (-sī, vowel lengthening, reduplication). 3rd-iō = mostly vowel lengthening. 4th = mostly -īvī (regular). When in doubt, look it up.
A dictionary lists a verb as capiō, capere, cēpī, captum. Which stem is captum showing you, and what gets built from it?
Study Tips
- •Drill the four parts out loud as a four-beat rhythm — amō, amāre, amāvī, amātum. The cadence is how Latinists actually store verbs.
- •Identify the conjugation from part 2 (the infinitive), not part 1. Capiō looks 4th but capere shows it is 3rd-iō.
- •Never guess the perfect from the present. Moneō → monuī (not monēvī); tegō → tēxī; fugiō → fūgī. Believe the dictionary.
- •Drop -um from part 4 and you have the supine stem — the source of both the perfect passive participle (amātus) and the future active participle (amātūrus).