The present subjunctive: forms

Build the subjunctive off the yo-form

The machinery is almost mechanical. Take the yo-form of the present indicative, knock off the final -o, and add the opposite family's endings: -ar verbs take the e-set, -er and -ir verbs take the a-set. Because the stem comes from the yo-form, most 'irregular' verbs are not irregular here at all. Tengo gives tenga, hago gives haga, conozco gives conozca.