How do you form the present subjunctive?

As with most verb conjugations, it's a question of applying the right patterns to the verb. I'll illustrate with porto, portare in both the active and the passive. To form the perfect subjunctive, you take the perfect stem (the part of the perfect tense form, minus the ending - portav-i -->portav; portatus sum -->portatus (remember to make the participle agree) and add the perfect ending relating to the person: active: erim, eris, erit, erimus, eritis, erint; passive: sim, sis, sit, simis, sitis, sint (this is the subjunctive present form of esse). And so: portaverim, portaveris, portaverit, portaverimus, portaveritis, portaverint portatus/a sim, portatus/a sim, portatus/a/um sit, portati/ae sint, portati/ae sitis, portati/ae/a sint