As all regular verbs are formed, the regular forms of the subjunctive mode of the present are formed by taking the regular root and adding the regular ending.
The regular subjunctive forms of the present tense are often described as using the 'opposite' endings.
Keep in mind that, in Spanish, usually the form to talk about myself is the same as the form to talk about someone else (he or she), and this is true for all the subjunctive forms.
Note that the subjunctive form of "da" and "doy" is "dé", with a written accent to distinguish it visually from the preposition "de". The other subjunctive forms don't have written accents: "den", "des", and "demos".
The indicative form of the verb often is found in the subordinate clause, but the opposite isn't true: the subjunctive form of the verb is rarely found in the main clause.
REGULAR FORMS - ROOT FIXES
(I.E., ORTHOGRAPHIC CHANGES)
Verb roots ending in C, G, and Z at times require spelling changes (orthographic changes) to maintain the same sound in all the forms.
REGULAR FORMS - ROOT VARIATIONS
(I.E., STEM CHANGES)
For the most part, verbs with root variations (a stressed variation of the root and an unstressed variation) do exactly what you expect.
But if the indicative form "we do" ends with "-imos", there is an extra unstressed root variation that shows up in the subjunctive form that ends with "-amos": dormimos → durmamos.
These are always verbs whose infinitive ends with "-ir".