The a-future, which is found in verbs belonging to one of the strong conjugation classes (B I to B V), is characterized by a reduplication of the initial consonant with the vowel i (usually lowered to e when the root vowel is a) as well as the suffix -ā- at the Proto-Celtic stage. The endings are thus basically identical to those of the a subjunctive. It lacks the suffix -s- of the s future, which is why it called an asigmatic future.
The initial consonant of the root is of course lenited after the i of the reduplicating syllable, and in most cases the vowel of the root has been syncopated away.
The future stem is used to form the conditional tense (called “secondary future” in some sources including Thurneysen and Strachan) as well as the future itself.
Some scholars, including Thurneysen and Strachan, refer to this as the reduplicated future, but as the s-future is also characterized by reduplication, that name is not specific enough.
See Category:Old Irish a future verbs for a list of verbs that take the a-future.
The basic pattern shows a reduplicating syllable with the vowel i:
The vowel of the reduplication syllable is lowered to e in:
In the verbs ernaid (“to bestow”) and aigid (“to drive”), the future stems are ebra- and ebla- respectively. These go back to Proto-Celtic formations *ɸibrā- and *ɸiblā-, where the b is from earlier ɸ before a liquid. Those two stems ebla- and ebra- were then extended to airid (“to plow”) and ailid (“to nourish”). Thus the following four verbs take eb- as the reduplicating syllable:
The é-future verbs (treated in a separate appendix) are a special class of a-future verbs in which the reduplicating vowel has been lengthened to é as a consequence of absorbing the following consonant. This pattern was reinterpreted as a replacement of the stem vowel with é and later became highly productive.
The future tense of téit has the suppletive stem rega- (from *rigā-, from *h₁ergʰ-: Ancient Greek ἔρχομαι (érkhomai)), which is not reduplicated.
The forms shown below for daimid (“to permit”) are constructed on the basis of attested endings.
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Absolute | didma | didmae | didmaid | didmaimmi | didmaithe | didmait | didmaithir | didmaitir |
| Conjunct | ·didam | ·didmae | ·didma | ·didmam | ·didmaid | ·didmat | ·didmathar | ·didmatar |
| Relative | didmas | didmaimme | didmaite | didmathar | didmaiter |
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural |
|---|---|---|---|---|---|---|---|
| ·didmainn | ·didmatha | ·didmad | ·didmaimmis | ·didmaithe | ·didmaitis | ·didmaithe | ·didmaitis |
No complete paradigm of the deponent conjugation is attested, but many endings can be deduced on the basis of other conjugations. Taking gainithir (“to be born”) as an example onto which attested and deduced endings can be added, we can derive the following:
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Absolute | gigniur | gignither | gignithir | gignimmir | gignithe | gignitir | — | — |
| Conjunct | ·gigniur | ·gignither | ·gignethar | ·gignemmar | ·gignid | ·gignetar | — | — |
| Relative | gignethar | gignemmar | gignetar | — | — |
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural |
|---|---|---|---|---|---|---|---|
| ·gigninn | ·gignithea | ·gigned | ·gignimmis | ·gignithe | ·gignitis | — | — |
The only deponent verb with an a-future for which passive forms are attested is ro·cluinethar. The future plural form is ·cechlaiter, which is expected; but the future singular form is ·cechlastar and the conditional singular form is ·cechlastai, both of which are s-future forms.