Why ir looks nothing like a regular -ir verb
A regular -ir verb such as vivir yields vivo, vives, vive in the present. Ir does not: the present stem is effectively v- (voy, vas, va...), the imperfect keeps an i- stem (iba...), and the preterite shares the fu- forms with ser. Spanish Linguist and historical grammars describe this as a highly irregular, partly suppletive paradigm rather than a normal -ir conjugation.
Learners hit two traps early. First, they try to invent forms like yo iro. Second, they meet fui and assume it can only belong to ser. Both ser and ir use fui, fuiste, fue in the preterite; Fui a Madrid is ir, Fui profesor* is ser. Read this page beside the ser table for that tense.
How to use the main tenses
Present (voy, vas, va...) states movement or plans: Voy al mercado. (I'm going to the market.) With a + infinitive it marks the near future: Voy a estudiar.
Preterite (fui, fuiste, fue...) reports a completed going: Fueron a Sevilla el lunes. Because the forms match ser, ask whether the sentence is about identity/essence (ser) or motion/destination (ir).
Imperfect (iba, ibas, iba...) describes ongoing or habitual going in the past: De niño iba a la playa cada verano. For the aspect choice between the two past tenses, see the dedicated comparison guide on this site.
Future (iré...) and conditional (iría...) are more regular in shape once you accept ir- as the stem: Iré mañana; Iría si pudiera.
Present subjunctive (vaya, vayas, vaya...) appears after wishes and doubts: Espero que vaya bien. The affirmative tú command is irregular ve (¡Ve a casa!); negative commands use the subjunctive (No vayas).
Ir vs ser in the preterite
| Sentence | Verb | Why |
|---|---|---|
| Fui a la tienda | ir | Destination / motion |
| Fui el capitán del equipo | ser | Identity in the past |
| Fue un buen día | ser | Characterization of the day |
| Fue a París en mayo | ir | Motion to a place |
If you can replace the verb with "went" in English, you want ir. If you can replace it with "was/were," you want ser. The Spanish forms alone will not tell you.
Non-finite forms and compounds
The infinitive is ir, the gerund yendo (not iendo), and the past participle ido. Compound tenses are haber + ido: he ido, había ido, habré ido, and the matching subjunctive compounds. RAE also still lists the rare future subjunctive (fuere) and preterite anterior (hube ido); you will mostly meet them in legal or older texts.
For the matching identity verb paradigm, stay with ser conjugation. Once you have the forms, use the preterite-versus-imperfect comparison to pick the right past tense.