The future passive infinitive says that an event will be undergone after the time of a governing verb: captum īrī “to be going to be captured.” Its form is unusual and comparatively rare: the accusative supine in -um plus the passive infinitive īrī. Neither part agrees with the person or thing affected.
All unlabelled uses below are possible in Classical prose, but the construction itself is chiefly a feature of careful indirect statement (formal/literary). Teaching forms use pedagogical macrons.
Formation: Supine in -um Plus īrī
Take the supine in -um, normally given as the fourth principal part, and add īrī, the present passive infinitive of eō “go.” Historically the expression is connected with movement toward carrying out an action; synchronically, Classical readers treat the whole phrase as a future passive infinitive.
| Principal parts | Supine | Future passive infinitive |
|---|---|---|
| amō, amāre, amāvī, amātum | amātum | amātum īrī |
| moneō, monēre, monuī, monitum | monitum | monitum īrī |
| regō, regere, rēxī, rēctum | rēctum | rēctum īrī |
| capiō, capere, cēpī, captum | captum | captum īrī |
| audiō, audīre, audīvī, audītum | audītum | audītum īrī |
Nūntiat oppidum captum īrī.
He reports that the town will be captured.
Crēdēbant lēgēs mūtātum īrī.
They believed that the laws would be changed.
The striking point in the second sentence is mūtātum, not mūtātās. The form in -um is a supine, not an agreeing perfect passive participle. It remains unchanged with a masculine, feminine, neuter, singular, or plural subject.
Posterior Time and Passive Voice
The event is passive because the embedded subject undergoes it, and future because it is later than the governing verb. As elsewhere in the infinitive system, the time relationship is relative; Future Infinitives in Indirect Statement places both voices in the wider reporting system.
Dīcunt pontem refectum īrī.
They say that the bridge will be rebuilt.
Dīxērunt pontem refectum īrī.
They said that the bridge would be rebuilt.
Latin leaves refectum īrī unchanged. English ordinarily changes “will” to “would” after a past report. That is an English sequence-of-tenses choice, not a change from future to conditional in the Latin.
Contrast perfect passive anteriority:
Dīcit pontem refectum esse.
He says that the bridge has been rebuilt.
Dīcit pontem refectum īrī.
He says that the bridge will be rebuilt.
Only the auxiliary distinguishes these superficially similar phrases: esse makes a perfect passive infinitive, while īrī makes a future passive infinitive. Translating both at first sight as “to have been” erases the very temporal contrast Latin marks.
The Accusative Subject, Without Agreement
In indirect statement the embedded subject is accusative, just as with other infinitives. What differs is agreement: the supine is frozen in -um.
Senātōrēs spērant captīvōs mox līberātum īrī.
The senators hope that the prisoners will soon be freed.
Rēx audīvit urbēs sociōrum dēfēnsum īrī.
The king heard that the allies’ cities would be defended.
Plural masculine captīvōs and plural feminine urbēs both take exactly the same supine form. This is the sharpest contrast with the future active infinitive: captīvōs discessūrōs esse has an agreeing future participle, but captīvōs līberātum īrī has an invariant supine.
An agent, if expressed, uses ā/ab with the ablative, as in other passive constructions:
Imperātor crēdit urbem ā mīlitibus servātum īrī.
The commander believes that the city will be saved by the soldiers.
The compact form is grammatically regular, but many Classical authors prefer a less compressed alternative, especially when several accusatives could make the clause hard to parse.
The More Flexible Alternative: Fore Ut
Latin commonly replaces a future passive infinitive with fore ut plus a passive subjunctive. The embedded ut clause can mark person and number, and it works even when a verb lacks a supine stem.
Spērō fore ut oppidum capiātur.
I hope that the town will be captured.
Spērābam fore ut oppidum caperētur.
I hoped that the town would be captured.
After present spērō, normal sequence gives present subjunctive capiātur; after imperfect spērābam, it gives imperfect caperētur. The future relationship comes from fore, not from a future subjunctive—Latin has no ordinary future subjunctive paradigm.
This alternative is not a makeshift error. It is idiomatic Classical prose (formal) and often clearer. When no fourth principal part exists, it is also necessary: never manufacture a supine merely to imitate English “will be.”
Rarity, Ellipsis, and Misleading Look-Alikes
The future passive infinitive is far rarer than the present and perfect passive infinitives. Learners need to recognize it accurately, but should not force it into every future passive report. Its compactness suits legal, historical, and rhetorical prediction (formal/literary).
Unlike esse with participial infinitives, īrī cannot normally be dropped: without it, the -um form is simply a supine whose construction must be supplied by something else. Conversely, factum īrī means “to be going to be done,” not “to have been done.” Perfect passive “to have been done” is factum esse.
The form īrī can also occur as the impersonal passive infinitive of eō in contexts about going, but after a transitive supine in -um, the combined future passive analysis is normally clear. Syntax outranks the first familiar gloss.
Common Mistakes
Making the supine agree
❌ Putant puellās vocātās īrī.
Incorrect — the future passive uses invariant supine vocātum, not an agreeing participle.
✅ Putant puellās vocātum īrī.
They think that the girls will be summoned.
Confusing esse and īrī
❌ Dīcit urbem captum īrī, meaning ‘He says the city has been captured.’
Incorrect meaning — īrī marks posterior time.
✅ Dīcit urbem captam esse.
He says that the city has been captured.
Using a nominative embedded subject
❌ Crēdit urbs dēfēnsum īrī.
Incorrect — the expressed subject of indirect statement must be accusative.
✅ Crēdit urbem dēfēnsum īrī.
He believes that the city will be defended.
Inventing a missing supine
❌ Spērō id futūrum īrī.
Incorrect as a manufactured future passive of sum; sum has no passive supine construction.
✅ Spērō fore ut id fīat.
I hope that it will happen / be done.
Key Takeaways
- The future passive infinitive is the invariant supine in -um plus īrī.
- It marks passive voice and time posterior to the governing verb.
- Its subject is accusative in indirect statement, but the supine does not agree with that subject.
- Fore ut plus passive subjunctive is an idiomatic and often more flexible Classical alternative.
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Start learning Latin →Related Topics
- Future Active InfinitiveA2 — How Latin combines a future active participle with esse to mark an active event later than its governing verb.
- Complementary InfinitivesA2 — How an infinitive completes the meaning of a modal, volitional, habitual, or phase verb while normally sharing its subject.
- Perfect Passive InfinitiveA2 — How participial agreement and esse express an event already undergone by the infinitive's subject.
- Accusative Subject of the InfinitiveA2 — Recognize the accusative noun or pronoun that serves as the subject inside a Latin infinitival clause.