The four verbs you meet first. Every form is shown, and the ones that break the regular rule are highlighted — so you can see how much of each verb is in fact regular.
to be: identity, origin, characteristics
Irregular in 7 of 17 tenses.
Ser shares its Indefinido, its Imperfecto de subjuntivo and its Futuro simple de subjuntivo with ir (fui, fuera, fuere), so context decides which verb is meant. Its Futuro simple and Condicional are regular (seré, sería), and so are all the compound tenses.
to be: states, location
Irregular in 6 of 17 tenses.
Estar keeps the regular -ar endings but has an irregular yo (estoy) and moves the written accent in the Presente (está, están) and the Presente de subjuntivo (esté, estén). It takes the stem estuv- in the Indefinido, the Imperfecto de subjuntivo and the Futuro simple de subjuntivo. The Imperfecto, Futuro simple and Condicional are regular.
to have, to hold
Irregular in 8 of 17 tenses.
Tener combines three irregularities: the stem change e → ie in the Presente (tienes), an irregular yo (tengo) that carries over to the whole Presente de subjuntivo (tenga), and the stem tuv- in the Indefinido and the Imperfecto de subjuntivo. The Futuro simple and Condicional use the shortened stem tendr-. In the Imperativo, tú is the short form ten. The Imperfecto is regular.
to go
Irregular in 7 of 17 tenses.
Ir builds its forms on three stems: v- in the Presente and Presente de subjuntivo (voy, vaya), i- in the Imperfecto, Futuro simple and Condicional (iba, iré, iría), and fu-, borrowed from ser, in the Indefinido and Imperfecto de subjuntivo (fui, fuera). In the Imperativo, tú is ve and nosotros is vamos; the negative nosotros form is no vayamos.
Most other irregular verbs follow a pattern shared by a whole family. The patterns page works through them.