How can you tell a third conjugation -io verb (like capio) from a fourth conjugation verb (like audio)?

By the infinitive: capere (short -e-) vs. audire (long -i-)

How is the future tense formed in the third conjugation?

With -am, -es, -et, -emus, -etis, -ent

Where does the -i- of third conjugation -io verbs (like capio) disappear?

Before a consonant and before short i, long i, and e (except in the future, participle, gerund, and gerundive)

Third Conjugation Paradigm: Latin Grammar

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GrammarThird Conjugation Paradigm

Third Conjugation Paradigm

A&G §186|6 rules|4 practice questions

The third conjugation is the wild one. Its stem-vowel was originally short -e- but it never sits still: regō, regis, regit, regimus, regitis, regunt — the vowel surfaces as i before most endings, as u before -nt.

The infinitive regere gives the conjugation away; the principal parts are regō, regere, rēxī, rēctum.

Two traps cost the most AP points. First, the future is regam, regēs, reget — looking just like the 1st-conj present subjunctive and nothing like the -bō, -bis future on either side.

Second, perfect formation is a free-for-all: -sī (dīxī, scrīpsī), bare -ī with vowel change (ēgī, vēnī), reduplication (cucurrī, cecidī), and -vī (petīvī). Principal parts must be learned verb-by-verb.

Pattern

PRINCIPAL PARTSregō, regere, rēxī, rēctum

PRESENTreg-ō, reg-is, reg-it, reg-imus, reg-itis, reg-unt

FUTUREreg-am, reg-ēs, reg-et, reg-ēmus, reg-ētis, reg-ent

IMPERFECTreg-ēbam, reg-ēbās, reg-ēbat…

Third Conjugation

Short-e stem-vowel that mutates to i and u; the future uses -am, -ēs (NOT -bō, -bis); perfect formation is open — learn each verb's principal parts.

The killer trap: applying the 1st-conj future -bō, -bis to a 3rd-conj verb. Regēbis does not exist — the future is regēs, the imperfect regēbās.

regō, regere, rēxī, rēctum — to rule, guide

Case	Active	Passive	Use
Pres. 1 sg.	reg-ō	reg-or	I rule / am ruled
Pres. 2 sg.	reg-is	reg-eris (	thematic vowel = i
Pres. 3 sg.	reg-it	reg-itur	thematic vowel = i
Pres. 1 pl.	reg-imus	reg-imur	thematic vowel = i
Pres. 2 pl.	reg-itis	reg-iminī	thematic vowel = i
Pres. 3 pl.	reg-unt	reg-untur	thematic vowel = u before -nt
Impf. 1 sg.	reg-ēbam	reg-ēbar	I was ruling — regular -ēbam
Impf. 2 sg.	reg-ēbās	reg-ēbāris	you were ruling
Impf. 3 sg.	reg-ēbat	reg-ēbātur	he was ruling
Fut. 1 sg.	reg-am	reg-ar	I shall rule — NOT regābō
Fut. 2 sg.	reg-ēs	reg-ēris (	you will rule — NOT regēbis
Fut. 3 sg.	reg-et	reg-ētur	he will rule
Fut. 1 pl.	reg-ēmus	reg-ēmur	we shall rule
Fut. 2 pl.	reg-ētis	reg-ēminī	you will rule
Fut. 3 pl.	reg-ent	reg-entur	they will rule — note u→e swap from regunt
Perf. 1 sg.	rēx-ī	rēctus sum	perfect stem rēx- (rēg-s-)
Perf. 3 sg.	rēx-it	rēctus est	he ruled / has ruled
Plup. 1 sg.	rēx-eram	rēctus eram	I had ruled
Fut. Perf. 1 sg.	rēx-erō	rēctus erō	I shall have ruled
Pres. infin.	reg-ere	reg-ī	to rule / to be ruled — short e
Pres. partic.	reg-ēns,	—	ruling
Perf. partic.	—	rēctus, -a,	having been ruled
Imper. sg.	reg-e	reg-ere	rule! / be ruled! — bare stem with short e

agō, agere, ēgī, āctum — to drive, do

Case	Active	Passive	Use
Pres. 1 sg.	ag-ō	ag-or	same present-system pattern as regō
Pres. 2 sg.	ag-is	ag-eris (	thematic vowel = i
Pres. 3 sg.	ag-it	ag-itur	he drives
Pres. 3 pl.	ag-unt	ag-untur	thematic vowel = u
Fut. 1 sg.	ag-am	ag-ar	I shall do — same -am, -ēs, -et
Fut. 2 sg.	ag-ēs	ag-ēris (	you will do
Impf. 1 sg.	ag-ēbam	ag-ēbar	I was doing
Perf. 1 sg.	ēg-ī	āctus sum	DIFFERENT perfect — bare -ī with vowel lengthening (a→ē)
Perf. 3 sg.	ēg-it	āctus est	he did / has done — no -s-, no -v-
Plup. 1 sg.	ēg-eram	āctus eram	I had done
Fut. Perf. 1 sg.	ēg-erō	āctus erō	I shall have done
Perf. infin.	ēg-isse	āctus esse	to have done / been done
Perf. partic.	—	āctus, -a,	having been done — supine stem āct-

Twelve High-Frequency 3rd-Conj Verbs by Perfect Type

-sī perfect — dīcō, dīcere, dīxī, dictum

dīxit "he said" — Caesar's go-to verb of speech

critical

-sī perfect — scrībō, scrībere, scrīpsī, scrīptum

scrīpsit "he wrote" — b → p before s

critical

-sī perfect — regō, regere, rēxī, rēctum

rēxit "he ruled" — the model verb itself

common

-sī perfect — mittō, mittere, mīsī, missum

mīsit "he sent" — double t simplifies before s

critical

-sī perfect — dūcō, dūcere, dūxī, ductum

dūxit "he led" — c + s = x

critical

bare -ī with vowel lengthening — agō, agere, ēgī, āctum

ēgit "he did" — a → ē in the perfect

critical

bare -ī with vowel lengthening — legō, legere, lēgī, lēctum

lēgit "he read / chose" — e → ē

important

bare -ī with vowel lengthening — vincō, vincere, vīcī, victum

vīcit "he conquered" — nasal infix drops in the perfect

important

reduplicating perfect — currō, currere, cucurrī, cursum

cucurrit "he ran" — root syllable repeated

common

reduplicating perfect — cadō, cadere, cecidī, cāsum

cecidit "he fell" — c-e-c- doubled root

common

-vī perfect — petō, petere, petīvī (petiī), petītum

petīvit "he sought" — looks 4th-conj, but isn't

important

-uī perfect — pōnō, pōnere, posuī, positum

posuit "he placed" — irregular perfect stem pos-

critical

See It In Action

Hinc portum petit, et socios partitur in omnes.

From here he heads for the harbor and divides his comrades among them all.

— Verg. Aen. i. 194

Petit shows the thematic vowel as -i- before -t — the stem is pet-, and the present 3rd sg. ending -it is the same shape every 3rd-conj verb wears here.

Imperium Dīdō Tyriā regit urbe profecta

Dido, having set out, rules the kingdom from her Tyrian city.

— Verg. Aen. i. 340

Here is the model verb in the wild: regit = "she rules" (present, thematic i). Trade the i for ē and you would have reget, "she will rule" — that single macron is the whole tense difference.

Caesar ad Lingonas litteras nuntiosque misit, nē eōs frūmentō neve aliā rē iuvārent

Caesar sent letters and messengers to the Lingones, telling them not to help the enemy with grain or anything else.

— B. G. i. 26

Mīsit is a -sī perfect: present mittō → mīsī (the double -tt- simplifies and s attaches to the root). This is the same family as dīxit, scrīpsit, rēxit, dūxit — the most common 3rd-conj perfect type.

Eō cum vēnisset, ea quae fore suspicātus erat facta cognōvit:

When he had arrived there, he learned that what he had suspected would happen had in fact happened:

— B. G. iv. 6

Cognōvit shows the -vī perfect type — built on the present stem with -ō-vī tacked on. Same conjugation as regō; entirely different perfect formation. This is why the principal parts are non-negotiable.

3rd-conj Future (regēs) vs. 1st-conj Future (amābis)

Students reflexively reach for -bō, -bis, -bit — the future of the conjugation they learned first. 3rd-conj uses a totally different paradigm.

1st conj. — *amābis*

future, 2 sg. — built with -bi-

amābis

Study Tips

•Drill the future regam, regēs, reget, regēmus, regētis, regent until it sounds wrong any other way — this is THE single most-missed paradigm on AP exams.
•When you meet a new 3rd-conj verb, memorize all four principal parts on day one. Unlike the 1st conjugation, you cannot guess the perfect from the present.
•Tell the present from the future by looking at the second person singular: regis (you rule, now) vs regēs (you will rule). The macron on the ē is the whole difference.
•Watch the 3rd plural present regunt and 3rd plural future regent — the u/e swap is tiny and easy to miss when reading at speed.

Third Conjugation Paradigm

See It In Action

Study Tips

Related Topics

Third Conjugation Paradigm

See It In Action

Study Tips

Related Topics