To form the present subjunctive of most verbs, add the personal endings of the present subjunctive to the yo form of the present indicative minus the -o ending. For the subjunctive, -ar verb endings are -e, -es, -e, -emos, -éis, -en, and -er / -ir verb endings are -a, -as, -a, -amos, -áis, -an. (Notice that the third person forms [él/ella/usted and ellos/ellas/ustedes] of regular verbs in the subjunctive are the same forms used in formal commands.)
 

-ar verbs

HABLAR
 

-er verbs

COMER
 

-ir verbs

VIVIR
 

Just as for the formal commands, since the form of the present subjunctive is taken from the yo form of the present indicative, stem-changing verbs and verbs that are irregular in the yo form have the same irregularity in the present subjunctive. In addition, verbs ending in -car, -gar, and -zar have the same spelling change that you learned for the formal commands: c > qu, g > gu, and z > c. (See the list of irregular and stem-changing verbs for the formal commands if you need to review these.)

The verbs that are irregular for the formal commands are also irregular for the present subjunctive. These forms must be memorized.

Present Subjunctive Forms—Irregular Verbs

These verbs are irregular in the present subjunctive and must be memorized:

DAR
 

ESTAR
 

HABER
 

SABER
 

IR