present subjunctive is based on the yo form of the present indicative.
There are six irregulars. They're irregular because they are the six verbs where the yo form of the present indicative doesn't end in an -o. for regulars you get the stem for the present subjunctive by dropping the o from the yo form of the present indicative, but for these six irregulars you have to memorize them separately.
Here they are, infinitives, yo form of the present indicative, and the stem for the present subjunctive
haber he hay-
saber sé sep-
ir voy vay-
ser soy se-
dar doy d-
estar estoy est-
Then add the endings, which are the same as for the regular verbs, except estar is weird.
haya, hayas, haya, hayamos, hayáis, hayan
sepa, sepas, sepa, sepamos, sepáis, sepan
vaya, vayas, vaya, vayamos, vayáis, vayan
sea, seas, sea, seamos, seáis, sean
dé, des, dé, demos, deis, den
esté, estés, esté, estemos, estéis, estén
Estar is weird because it's stressed on the last syllable in the boot, which is unusual, like I think it's the only verb that does that? It's probably because it comes from latin stare, and then you drop the final -e and get "star", but then spanish has a rule about S's so you have to put an e before it for pronunciation, so it's estar. Anyway, the endings for estar in the present subjunctive are regular except for the stress pattern.
dar is actually regular in the forms of the present subjunctive. The only reason it's considered irregular is that the yo form of the present indicative doesn't end in an o, so it technically doesn't fit the rule of dropping the o to get the stem for the present subjunctive. You have to drop the -oy.
ser is almost regular, the only problems being that you have to drop the -oy from the yo form of the present indicative instead of just the -o, like dar, and also that the present subjunctive has an extra "e" hanging out in its stem, whereas if it were regular the stem would just be s- like how the stem for dar is d-.
ver and haber are weird, idk where their present subjunctive forms come from.
the present subjunctive of saber is based on an old form of the yo form of the present indicative: sepo. There's like one other verb that still has an irregular yo form of the present indicative like that and that is caber (quepo). since the yo form of the present indicative of caber IS currently quepo, the present subjunctive of caber is not irregular, it's just based directly on quepo. Saber is irregular because it's based on sepo, but the modern yo form of the present indicative of saber is sé.
and like i said for all the regular ones you drop the o from the yo form of the present indicative and add the endings listed above, but that's not all! There are stem-changes to think about.
member how some stem changes appear in the boot and some appear only in the short-boot? If a verb is stem-changing in the present indicative but only in the short-boot, then the stem-change does NOT appear in the present subjunctive. So you just add the endings and you're done.
If the stem-change appears in the yo form of the present indicative, then it will appear in the BOOT of the present subjunctive.
And then you have to check to see if it's an ar, er, or ir verb. If it's an ar or er verb, you're done. If it's an ir verb, then a *reduced stem-change* appears in the NOT-BOOT, ie the nosotrxs and vosotrxs forms. The reduced stem-change is e -> i and o -> u. (That means if the verb's stem-change was e -> i to begin with then all six forms end up with the same e -> i stem-change. There's no such thing as an o -> u stem change so that's not an issue. If it was i -> ie, then the reduced stem-change looks exactly like a lack of a stem-change. That's rare but does occur. And there is such a thing as u -> ue stem-change, but there's only one verb that does that and it's not an -ir verb, so it's not an issue here either.)
So I think that's all.