Yo-form stem (Presente / Subjuntivo)
The presente de subjuntivo is built on the yo form of the presente, so whatever is irregular in yo runs through the whole subjunctive: tengo → tenga, conozco → conozca, hago → haga, construyo → construya.
The card. The back shows the yo form of the Presente and of the Presente de Subjuntivo, one under the other. The book opens the verb's conjugation at those two tenses.
The deck. Every verb in your selection whose subjunctive comes from an irregular yo form: the verbs in -go, -zco and -uir, and those with a yo form of their own. Dar, estar, haber, ir, saber and ser are not in it — their subjunctive is not built this way — and neither are the stem-changing verbs, which have Stem changes (Presente). About one card in five is a regular verb — twenty irregulars bring five — drawn afresh each time you open the drill, so you have to decide whether the verb departs at all.
Grading and the session work as in Future stem (Futuro / Condicional), and each graded card is recorded once for every form on its back.