Īnfīnītīvus · Participium · Gerundium · Supīnum

The infinitive (īnfīnītīvus) is a verbal noun — neuter in gender, indeclinable, naming the action without person or number. It is etymologically a "non-finite" form (Lat. īnfīnītus "unbounded") because it does not specify subject person or number, only the action itself.

Latin has six infinitives, distributed across present, perfect, future tenses and active and passive voices:

(1) Present active — amāre, monēre, regere, capere, audīre "to love, warn, rule, seize, hear"; for sum: esse.
(2) Present passive — amārī, monērī, regī, capī, audīrī "to be loved...". 1·2·4 conj. add -rī; 3·3-iō conj. just -ī.
(3) Perfect active — amāvisse, monuisse, rēxisse, cēpisse, audīvisse "to have loved..."; fuisse for sum.
(4) Perfect passive — periphrastic: perfect passive participle + esse: amātus esse "to have been loved".
(5) Future active — periphrastic: future active participle + esse: amātūrus esse "to be about to love"; futūrum esse or fore for sum.
(6) Future passive — supine + īrī (rare): amātum īrī "to be about to be loved". This is a fossil; classical writers prefer fore ut + subj.: fore ut amētur.

The infinitive serves three principal syntactic functions: as subject of an impersonal verb; as object/complement of certain verbs; as the verb of indirect statement (with accusative subject — see Lectiō XXXII).

Infinitive as subject: with impersonal verbs (licet, oportet, decet, libet, placet, iuvat, opus est, necesse est), the infinitive serves as logical subject. The infinitive's "subject" (the person performing) appears in the accusative or dative depending on the verb:

licet mihi īre "to-go is permitted to me / I may go" (dative person, infinitive subject).
oportet tē venīre "to-come is incumbent on you / you ought to come" (accusative person, infinitive with subject).

Infinitive as complement (object): completes the meaning of certain verbs of ability, willingness, beginning, ceasing, daring, knowing-how, learning, being able. The subject of the infinitive is implicit (= subject of main verb).

Common verbs taking complementary infinitive: possum (be able), volō (wish), nōlō (be unwilling), mālō (prefer), cōnor (try), cupiō (desire), incipiō (begin), dēsinō (cease), audeō (dare), statuō (decide), cōnstituō (resolve), sciō (know how), discō (learn), doceō (teach), iubeō (order — with acc.+inf.), parō (prepare), dubitō (hesitate).

Examples: fugere possum "I can flee"; scrībere volō "I wish to write"; cōnātur dīcere "he is trying to speak"; incēpit cantāre "he began to sing".

Indirect statement: after verbs of saying, thinking, perceiving, knowing, etc., the reported clause becomes accusative + infinitive. The original verb's subject becomes accusative; the verb itself becomes infinitive (with relative tense). Fully treated in Lectiō XXXII.

The participle (participium) is a verbal adjective, agreeing with its noun in gender, number, and case while retaining the verb's tense, voice, and ability to take objects/complements. Latin has four participles, distributed across active/passive and tense:

(1) Present active — amāns, -antis "loving"; declines like prūdēns (3rd-decl., one-ending).
(2) Future active — amātūrus, -a, -um "about to love, going to love"; declines like bonus.
(3) Perfect passive — amātus, -a, -um "(having been) loved"; like bonus.
(4) Future passive (gerundive) — amandus, -a, -um "to be loved, requiring loving"; like bonus. Also functions adjectivally as the gerundive (treated separately below).

Latin lacks:
• Present passive participle (no equivalent of "being loved").
• Perfect active participle (deponent verbs supply this gap with their perfect participles, which are passive in form but active in meaning).

The two missing participles must be periphrasised in Latin: "while loving (passive)" → cum amārētur; "having loved" → cum amāvisset; or restructured with relative clauses or perfect tenses.

The deponent verbs are useful precisely because they fill these gaps: locūtus "having spoken" is morphologically passive but functions as the perfect active participle that ordinary verbs lack.

The participle's tense is relative to the main verb: present participle = simultaneous with the main action; perfect participle = prior; future participle = subsequent.

Caesar veniēns vidit hostēs "Caesar, (while) coming, saw the enemy" (simultaneous, present participle).
Caesar oppidum captum incendit "Caesar burned the captured town" (prior, perfect passive).
moritūrus tē salūtō "(I) about to die salute you" (subsequent, future active — gladiator's salute).

Participles can serve multiple syntactic roles:

(a) Attributive: like ordinary adjectives, modifying a noun: vir loquēns "a speaking man"; oppidum captum "the captured town".

(b) Substantival: used as a noun: amāns "a lover"; moriēns "a dying person"; capta (f.) "a captive woman"; fugientēs "the fleeing (men)".

(c) Predicative: with a linking verb or as predicate: fessus est "he is tired" (fessus as adj.); amātus est "he was loved" (forming the perfect passive periphrasis).

(d) Adverbial: equivalent to a temporal, causal, concessive, or conditional clause: territī fūgērunt "frightened, they fled" (= "because they were frightened" or "after they had been frightened").

(e) Ablative absolute: an entire clause built around a participle in the ablative; see Lectiō XXVIII. urbe captā, hostēs fūgērunt "the city having been captured, the enemy fled".

The future active participle is the basis of the active periphrastic conjugation: amātūrus sum "I am about to love"; amātūrus eram "I was about to love". This conjugation is somewhat rare and emphatic.

The gerund (gerundium) is a verbal noun — neuter, second declension — supplying the cases that the present infinitive cannot take. The gerund exists only in four cases (genitive, dative, accusative, ablative) and only in the singular; it has no nominative (the present infinitive supplies that role: amāre est dulce "to love is sweet").

Forms: stem + -nd- + 2nd-decl. neuter endings (-ī, -ō, -um, -ō):
I conj.: amandī, amandō, amandum, amandō.
II conj.: monendī... III conj.: regendī... III-iō: capiendī... IV: audiendī...

The gerund retains the verb's ability to take an object — and this is where it competes with the gerundive (next section).

Common uses:

(a) Genitive: with nouns expressing causation/purpose: cupiditās discendī "desire of/for learning"; ars scrībendī "the art of writing"; tempus pugnandī "time for fighting".

(b) Dative: with adjectives of fitness, also for purpose: aptus pugnandō "fit for fighting"; sōlemne sacrificandō "for sacrificing".

(c) Accusative: only with ad for purpose: ad discendum "for the purpose of learning, in order to learn"; ad pugnandum "for fighting".

(d) Ablative: of means: discendō discimus "we learn by learning"; or with prepositions in, ex, dē: in legendō "in (the act of) reading"; ex audiendō "from listening"; dē hoc loquendō "from speaking about this".

The gerundive (gerundīvum) is identical in form to the gerund but is a passive verbal adjective — agreeing with a noun in gender, number, and case. It carries the sense "to be (X-ed)", "deserving (X)", or expressing necessity / obligation.

amandus puer "a boy to be loved / a lovable boy"; liber legendus "a book to be read / worth reading".

The gerundive substitutes for the gerund whenever the gerund would govern an accusative direct object. This is the most distinctive feature:

Substitution rule: instead of cupiditās librum legendī ("desire of reading a book", lit. "of-reading [acc.] a book"), Latin prefers cupiditās librī legendī ("desire of a book to-be-read", with the noun in the genitive and the gerundive agreeing).

Both constructions mean the same thing, but the gerundive construction is decisively more common in classical prose. The gerund-with-direct-object construction is found chiefly in poetry and early Latin.

Passive periphrastic conjugation: gerundive + form of sum = "must be (X-ed)", with the agent in the dative:

liber legendus est mihi "the book must be read by me / I must read the book".
cōnsulendum est tibi "you must take counsel".
Carthāgō dēlenda est "Carthage must be destroyed" (Cato's famous closing of every senate speech).

This is the standard Latin idiom for obligation, central to Cicero's ethical and political prose.

The supine (supīnum) is a verbal noun of the fourth declension, surviving in only two cases: accusative singular in -um and ablative singular in -ū. It is the fourth principal part of every Latin verb:

amātum, amātū; monitum, monitū; rēctum, rēctū; captum, captū; audītum, audītū.

Accusative supine (in -um): expresses purpose after verbs of motion. vēnit oppidum oppugnātum "he came to attack the town" (lit. "he came [for the purpose of] attacking the town"). The construction is rare in classical prose; ad+gerund/gerundive or ut+subjunctive ordinarily replaces it. It survives mainly in:
• Stock idioms: cubitum īre "to go to bed"; spectātum venīre "to come to watch (a show)"; petītum venīre "to come to seek".
• The future passive infinitive periphrasis: amātum īrī.

Ablative supine (in -ū): used after a few adjectives ("ablative of respect/specification") and with opus est:
• mīrābile dictū "wonderful to say (= in the speaking)";
• facile factū "easy to do (= in the doing)";
• turpe audītū "disgraceful to hear";
• incrēdibile vīsū "incredible to see".
• scītū opus est "it is worthwhile to know".

The ablative supine is more frequent than the accusative in classical Latin; both have an archaic flavor and concentrate in specific stylistic registers.