Present Subjunctive Rule

Question

Proteger becomes protejo in first person present tense. Can’t you pretty much always dro the o and have the stem for the present subjunctive? For me, it is easier to remember.

Answer

Hi there Stan ... Yes, you can often do as you suggest^ ... There are, however, a lot of verbs where the stem of the subjunctive for nosotros and vosotros is slightly different from that used for yo, tu, él and ellos:

e.g. sentir > siento > yo sienta [subj.] - but > sintamos [= subj. for 'we'];

and dormir > duermo > yo duerma [subj.] - but > durmamos [= subj. for 'we']

and morir > muero > yo muera [subj.] - but muramos [=subj. for 'we']...[< Thank you Inma !

Other '-ir' verbs with that^ nosotros irregularity include ones ending in '-erir' [preferir, sugerir, etc. etc.] or ending in '-vertir' or in '-entir' [mentir, arrepentirse etc.]

Also, a lot of '-er' verbs show the same sort of 'misbehaviour' - e.g. mover, morder, oler, volver, perder etc. etc.

Plus a good number of '-ar' verbs, e.g. cerrar, confesar, pensar, contar, acostarse, mostrar, probar etc. etc.

// Sometimes the nosotros stem-vowel change in the subjunctive is the same as that adopted in the indicative - [but not always: e.g. with sentir].

Answer

Hola Stan
We do take the first person (yo) conjugation of the present tense as the base to form the present subjunctive conjugations, but as David said, we can't apply that rule to all verbs as there are so many irregularities to consider. It's good to remember that the verbs with a stem change (e>ie, e>i and o/u>ue) in the present tense indicative have (nearly 100% of the time) the same irregularity in the present subjunctive. Apart from the stem changing verbs already mentioned, we need to consider consonant changes too. See for example "empezar", with a change from z to c:
empezar 
empiezo, empiezas, empieza, empezamos, empezáis, empiezan
empiece, empieces, empiece, empecemos, empecéis, empiecen
(Just a little correction to what David wrote:  "morimos [=subj. for 'we']" -> what David meant was: "muramos [=subj. for 'we']")
Saludos
Inma