Complementary Infinitives

A complementary infinitive completes the meaning of a finite verb that is semantically unfinished on its own: “can do,” “wants to go,” “begins to speak.” In Mārcus venīre potest, Marcus is both the person who can and the person who comes. That shared-subject structure—not the presence of English “to”—is the central diagnostic.

All unlabelled uses below are ordinary Classical prose. Teaching examples use macrons; the source quotation preserves conventional unmacronized spelling.

Which Verbs Take a Complementary Infinitive?

The most frequent governing verbs express ability, obligation, desire, preference, attempt, habit, or the beginning and ending of an action.

The infinitive completes each governing idea. English alternates among a bare infinitive (“can work”), a to-infinitive (“wants to learn”), and sometimes an -ing form (“begins speaking”). Latin does not copy that surface variation: the same infinitive category serves throughout.

There is still lexical selection to learn. English “persuade someone to go” is normally Latin alicui persuādeō ut eat, with a dative person and an ut clause—not a complementary infinitive. There is no universal rule that every English verb followed by “to” takes a Latin infinitive.

One Subject Controls Both Actions

With the ordinary complementary pattern, the nominative subject of the finite verb is also the understood subject of the infinitive. Latin does not repeat it in the accusative.

The first sentence has a complementary infinitive: Marcus wants and Marcus comes. The second contains an accusative-and-infinitive clause: Marcus wants, but Lucia comes. The accusative Lūciam signals the change of embedded subject.

This contrast can affect reflexives and objects as well. In Iūlia sē dēfendere potest, Julia can defend herself; sē is the object of dēfendere, not a separately expressed subject. In Iūlia eum discēdere vult, eum is the subject of “leave,” not the object of “want.” First determine the semantic participants, then assign cases.

For the distinct-subject construction, see Accusative Subject of the Infinitive.

Time: Usually Simultaneous or Prospective by Context

The present infinitive is overwhelmingly normal after modal, volitional, and phase verbs. It does not mean that the action is happening at speech time. Instead, it names the action whose possibility, desire, obligation, or phase is located by the finite verb.

The present infinitive proficīscī works with future reference in the first sentence and an unrealized past intention in the second. English-speaking learners often reach for a future infinitive because the departure is later than the wanting. Latin treats volō + infinitive as one modal complex; the adverb and finite tense establish the timeline.

A perfect infinitive can occasionally be licensed where completed action is genuinely in view, and poets extend such patterns (literary), but it is not the default way to say “wanted to have done.” Do not replace ordinary crās venīre volō with ventūrum esse volō. The latter is a cumbersome participial proposition, not the neutral complementary construction.

Active and Passive Infinitives

The infinitive’s voice reflects the role of the shared subject. If the subject performs the action, use an active infinitive; if it undergoes the action, use a passive infinitive.

In both sentences discipulī is nominative because it is the subject of volunt. In the first, it is also the agent of audīre; in the second, it is the patient of audīrī. English and Latin align here, but Latin form makes the voice contrast particularly compact.

A passive infinitive can also turn an inanimate nominative into the shared patient:

Latin literally structures this as “this letter is-able to-be-read.” English normally uses impersonal “can be,” but plural-form litterae is grammatically the nominative subject of possunt. The finite verb agrees with it.

Phase Verbs and Special Behavior

Incipiō has a full conjugation and means “begin.” Coepī is defective: it supplies perfect-system forms, so loquī coepit means “she began to speak.” Dēsinō means “stop, cease.” These verbs take complementary infinitives in ordinary prose.

With a passive infinitive, Classical Latin often makes coepī itself passive and lets its participle agree with the shared subject: mūrī reficī coeptī sunt, literally “the walls were begun to be repaired.” This construction is normal but more characteristic of formal historical prose (formal/literary) than of elementary composition.

The wording is not a perfect passive of reficiō. Coeptī sunt is the finite perfect passive of coepī and agrees with mūrī by a regular Classical attraction to the passive infinitive; reficī supplies the action begun.

The Vulgate preserves a very clear, later Latin example (Luke 14:30):

Its spelling is source-faithful and therefore unmacronized. Both aedificare and consummare complement their governing verbs. The construction is continuous with Classical usage, although the quotation’s register is (late/biblical) rather than Classical prose.

Infinitive Complement or Infinitive as Subject?

Some impersonal expressions take an infinitive clause: licet mihi abīre “I am permitted to leave,” necesse est abīre “it is necessary to leave.” Traditional descriptions may call these complementary, but syntactically the infinitive is better understood as a subject-like clause, while mihi is a dative experiencer. The shared nominative-subject test does not apply because the governing expression is impersonal.

That distinction matters less for translation than for case choice. Latin says mihi licet, not nominative ego licet. The dedicated Infinitive as Subject page treats that structure fully.

Common Mistakes

Repeating the shared subject in the accusative

Copying English to after every governing verb

Moving futurity to the governing verb merely because the action is later

Ignoring the voice of the shared subject

Key Takeaways

• A complementary infinitive completes a finite verb such as possum, volō, dēbeō, soleō, or incipiō.
• The finite verb and infinitive normally share one subject; do not add an accusative subject unless a distinct actor is intended.
• The present infinitive is normal even when context places the desired or possible action in the future.
• Choose active or passive voice according to whether the shared subject performs or undergoes the infinitive’s action.
• Learn which Latin construction each governing verb selects instead of copying the English word “to.”