The irregularity of the verb is in its stems which are most often a single vowel. Once you are familiar with the stems, the conjugation of the verb is straightforward.
Note: Active forms of the verb are very common and part of the core vocabulary of Latin. Passive forms of the verb are rare and very restricted in their usage. Therefore, all passive forms will be discussed separately.
[i] Present tense
eō [stem: e- + personal ending: ō]
is [stem: i- + personal ending: s]
it [stem: i- + personal ending: t]
īmus [stem: ī- + personal ending: mus]
ītis [stem: ī- + personal ending: tis]
eunt [stem: eu- + personal ending: nt]
[ii] Imperfect and future tenses
single stem vowel: ī- + 1st / 2nd imperfect tense and future tense endings:
- imperfect: ībam, ībās etc.
- future: ībō, ībis etc.
[iii] Perfect tense stems
[a] single vowel: i- + perfect tense endings
However, note the spelling changes marked with an asterisk:
(1) iī [perfect stem i- + perfect tense ending -ī]
* (2) īstī [perfect stem i- + perfect tense ending -istī, but the two vowels merge > īstī]
Note: iistī is sometimes attested, especially in the work of Plautus:
- Strēnuē mehercle iistī.
- Bis periistī?
- Iamne abiistī?
(3) iit [perfect stem i- + perfect tense ending -it]
(4) iimus [perfect stem i- + perfect tense ending -imus]
* (5) īstis [perfect stem i- + perfect tense ending -istis, but the two vowels merge > īstis]
Note: iistis is sometimes attested, especially in the work of Plautus:
- Iam vōs rediistis in concordiam?
- Ā mē magnam iniistis grātiam.
(6) iērunt [perfect stem i- + perfect tense ending -ērunt]
[b] perfect stem: īv- + the perfect tense endings
These forms do not occur as often with the verb eō, īre; in Classical literature, some do not exist at all, but are used in later Latin. However, they are used as perfect tense endings for 4th conjugation verbs throughout the Classical period.
īvī | audīvī
īvistī | audīvīstī
īvit | audīvit
īvimus | audīvimus
īvistis | audīvistis
īvērunt | audīvērunt
[iv] Pluperfect and future perfect tenses
[a] perfect stem: i- + pluperfect / future perfect endings
[b] perfect stem: īv- + pluperfect / future perfect endings
pluperfect: ieram, ierās etc. | īveram, īverās etc.
future perfect: ierō, ieris etc. | īverō, īveris etc.
