Preterite stem (Indefinido / Imperfecto de subjuntivo)
The imperfecto de subjuntivo is built on the ellos form of the indefinido — drop -ron, add -ra — so every irregularity of the indefinido carries over: tuvieron → tuviera, pidieron → pidiera, leyeron → leyera, fueron → fuera.
The card. The back shows the ellos form of the Pretérito Indefinido and the yo form of the Imperfecto de Subjuntivo, each labelled with its tense and person. The book opens the verb's conjugation at those two tenses.
The deck. Every verb in your selection whose ellos form in the indefinido is irregular: the strong preterites, the -ir verbs whose vowel changes in the third person, the verbs that write a y, and those with forms of their own. Verbs that only change their spelling in the yo form (busqué) are not in it: their ellos form is regular. 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.