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
hable	hablemos
hables	habléis
hable	hablen

 

-er verbs
COMER
coma	comamos
comas	comáis
coma	coman

 

-ir verbs
VIVIR
viva	vivamos
vivas	viváis
viva	vivan

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
dé	demos
des	deis
dé	den

 

ESTAR
esté	estemos
estés	estéis
esté	estén

 

HABER
haya	hayamos
hayas	hayáis
haya	hayan

 

SABER
sepa	sepamos
sepas	sepáis
sepa	sepan

 

IR
vaya	vayamos
vayas	vayáis
vaya	vayan