Formation of the infinitive

To form the infinitive, use the verbs stems you already know: first principal part for the present infinitives, third and sixth principal parts for the aorist infinitives, and fourth and fifth principal parts for the perfect infinitives.

Thematic (-ω) verbs

For regular thematic (-ω) verbs, apply the following endings:

Tense	Active	Middle	Passive
Present	-ειν	-εσθαι	-εσθαι
First aorist	-αι	-ασθαι	-ῆναι
Second aorist	-εῖν	-έσθαι	-ῆναι
Perfect	-έναι	-σθαι	-σθαι

Since infinitives are a kind of noun form, they have persistent accent. You can see this clearly in the following set of infinitives for the verb κελεύω: you can see from their accent that the aorist and perfect active infinitives, the aorist passive, and the perfect middle and active infinitives are not recessive.

Tense	Active	Middle	Passive
Present	κελεύειν	κελεύεσθαι	κελεύεσθαι
Aorist	κελεῦσαι	κελεύσασθαι	κελευσθῆναι
Perfect	κεκελευκέναι	κεκελεῦσθαι	κεκελεῦσθαι

As you have seen throughout the finite and participle forms you already know, second aorists use the same endings as forms from the present system. The following chart illustrates the infinitives of βάλλω (third principal part: ἔβαλον). Notice that the accent of the second aorist active and middle persists on the ending, in contrast to the present active and middle infinitives. (This can be a helpful clue if you encounter a new verb and don’t recognize its principal parts.)

Tense	Active	Middle	Passive
Present	βάλλειν	βάλλεσθαι	βάλλεσθαι
Aorist	βαλεῖν	βαλέσθαι	βληθῆναι
Perfect	βεβληκέναι	βεβλῆσθαι	βεβλῆσθαι

Of course for verbs that are deponent in one or more principal parts (that is, that do not have active forms), there is no active infinitive. γίγνομαι, for example, has no active forms in the present or aorist (third principal part ἐγενόμην). These stems give us the present middle infintive γίγνεσθαι, the (second) aorist middle infinitive γενέσθαι.

Athematic (-μι) verbs

As with finite verb forms, the present system of athematic verbs uses a slightly different set of endings (without the thematic, or transitional, vowel between the stem and ending). The present infinitive endings of athematic verbs are:

Tense	Active	Middle	Passive
Present	-ναι	-σθαι	-σθαι

Outside of the present system, regular athematic verbs like δείκνυμι form the aorist and perfect tenses the same way that thematic verbs do (again, just as the finite forms do), using the third and sixth parts for the aorist, the fourth and fifth parts for the perfect. Applying the appropriate endings then gives us these infinitives for δείκνυμι:

Tense	Active	Middle	Passive
Present	δεικνύναι	δείκνυσθαι	δείκνυσθαι
Aorist	δεῖξαι	δείξασθαι	δειχθῆναι
Perfect	δεδειχέναι	δεδεῖχθαι	δεδεῖχθαι

And of course deponent athematic verbs will have no active infinitives. The verb δύναμαι, for example, is an athematic verb with only passive forms. From the first principal part δύναμαι, we drop the athematic ending -μαι and add the ending -σθαι direcelty to the stem δυνα-. We use the fifth and sixth parts δεδύνημαι and ἐδυνήθην just as we would for any thematic or athematic verb, so that we have the following set of passive infinitives for δύναμαι:

Tense	Passive
Present	δύνασθαι
Aorist	δυνηθῆναι
Perfect	δεδυνῆσθαι

Epsilon contract verbs

Recall these patterns of contraction that you saw in the finite forms of epsilon contract verbs:

ε + ει => ει
ε + ε => ει

These rules produce the following entirely predictable infinitive forms:

Tense	Active	Middle	Passive
Present	ποιεῖν	ποιεῖσθαι	ποιεῖσθαι
Aorist	ποιῆσαι	ποιήσασθαι	ποιηθῆναι
Perfect	πεποιηκέναι	πεποιῆσθαι	πεποιῆσθαι

Alpha contract verbs

In finite verb verbs, we normally expect α + ει => ᾳ (with iota subscript); in infinitive forms, however, the pattern is

α + ε => α

That rule give us τιμᾶν for the present active infinitive, τιμᾶσθαι for the present middle and passive infinitives. The aorist and perfect forms are built normally from the third through sixth principal parts.

Tense	Active	Middle	Passive
Present	τιμᾶν	τιμᾶσθαι	τιμᾶσθαι
Aorist	τιμῆσαι	τιμήσασθαι	τιμηθῆναι
Perfect	τετιμηκέναι	τετιμῆσθαι	τετιμῆσθαι

Irregular verbs

Learn the present active infinitives of these three extremely common irregular verbs:

εἰμί (“to be”): present active infinitive εἶναι
εἶμι (“to go”): present active infinitive ἰέναι
φημί (“to say”): present active infinitive φάναι