Stem changes (Presente)
A stem-changing verb changes its stem vowel wherever the stress falls on it — yo, tú, él and ellos — and keeps the infinitive's vowel in nosotros and vosotros. In a table the four changed forms make the shape of a boot: quiero, quieres, quiere, queremos, queréis, quieren. The changes are e → ie, o → ue, e → i, and u → ue (jugar) and i → ie (adquirir).
The card. The back shows the whole Presente, one row per person, labelled with its pronoun. The book opens the verb's conjugation at the presente.
The deck. Every stem-changing verb in your selection. Verbs whose yo form has an irregularity of its own (tener → tengo) are in Yo-form stem (Presente / Subjuntivo) instead. 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.