Formation of the present subjunctive

You form the presente de subjuntivo almost always from the yo-form of the present indicative.

Steps:
- Take the yo-form (e.g. hablo, como, vivo)
- Remove the -o → habl-, com-, viv-
- Add the 'opposite' ending: -ar verbs get -e endings, -er/-ir verbs get -a endings

That's why the subjunctive form of an -ar verb looks like the indicative of an -er verb, and vice versa.

Verbs that have a stem change in the present indicative (e→ie, o→ue, e→i) keep that change in the subjunctive too, in the same forms: all forms except nosotros and vosotros.

For stem-changing -ir verbs (like pedir, dormir, sentir), the stem also changes in nosotros and vosotros, but differently: e→i or o→u.

Some verbs change their spelling to keep the stem's pronunciation:

• -car → -que: buscar → busque
• -gar → -gue: llegar → llegue
• -zar → -ce: empezar → empiece
• -ger/-gir → -ja: coger → coja, elegir → elija
• -guir → -ga: seguir → siga

These changes apply to every form of the paradigm, because the ending always starts with -e or -a.

A small group of verbs doesn't follow the yo-form rule and simply has to be memorized:

• ser → sea
• estar → esté
• ir → vaya
• haber → haya
• saber → sepa
• dar → dé (with an accent, to distinguish it from the preposition de)

These six verbs are extremely common, so it's worth learning the full conjugation.