The subjunctive form (present) 1

Forming the present subjunctive from the yo form — Take the yo present indicative, drop the -o, and add opposite-vowel endings: -ar → -e, -es, -e, -emos, -éis, -en; -er/-ir → -a, -as, -a, -amos, -áis, -an. Hablo → habl-: hable, hables…; como → com-: coma, comas… Any yo irregularity carries through the whole subjunctive.

Yo-form irregulars create subjunctive irregulars — Because the subjunctive is built on the yo form, go-verbs are irregular throughout the subjunctive: tener → tengo → tenga, tengas…; hacer → hago → haga, hagas…; poner → pongo → ponga…; decir → digo → diga…; salir → salgo → salga…

Six verbs with irregular subjunctive stems — These six don't derive from the yo form and must be memorised: ser → sea, estar → esté, ir → vaya, dar → dé, saber → sepa, haber → haya. These same six are the most common irregular subjunctive verbs and appear constantly in everyday Spanish.