In short

Many common verbs keep the regular endings from Lesson 5 but change their stem - either a vowel in the stem (quiero, puedo) or just the yo form (tengo, conozco). A few verbs are irregular all through.

The rule

Group 1 - Stem-changing “boot” verbs. The stressed vowel of the stem changes in every form except nosotros and vosotros (there the stress moves onto the ending, so the stem stays as it is). If you draw a line around the four changed forms, it looks like a boot.

• e -> ie: querer, pensar, empezar, entender, preferir. quiero, quieres, quiere, queremos, queréis, quieren.
• o -> ue: poder, volver, dormir, contar, encontrar. puedo, puedes, puede, podemos, podéis, pueden.
• e -> i (only -ir verbs): pedir, servir, repetir. pido, pides, pide, pedimos, pedís, piden.
• u -> ue (only jugar): juego, juegas, juega, jugamos, jugáis, juegan. A game or sport takes a: juego al tenis, jugamos a las cartas - but a place takes en (los niños juegan en el patio), and a match or a round is a plain object, with no preposition at all: jugamos la final.

Group 2 - Irregular yo only. These are regular everywhere except the yo form:

• -go: hacer -> hago, poner -> pongo, salir -> salgo, traer -> traigo. Some also stem-change: tener -> tengo (tienes…), venir -> vengo (vienes…), decir -> digo (dices…).
• -zco (verbs in -cer/-cir after a vowel): conocer -> conozco, conducir -> conduzco, parecer -> parezco, traducir -> traduzco.
• Odd ones: saber -> sé, dar -> doy, ver -> veo, caber -> quepo.

Group 3 - Wholly irregular. Learn these as whole sets: ser (soy, eres…), estar (estoy, estás…), ir (voy, vas, va, vamos, vais, van), haber (he, has, ha… / hay), oír (oigo, oyes, oye, oímos, oís, oyen).

saber vs conocer

Both mean “to know”, and both have an irregular yo:

• saber = to know facts/information, or to know how to do something (saber + infinitive). Sé la respuesta. Sé nadar.
• conocer = to be acquainted with a person, place, or thing. Conozco a María. Conozco Madrid.

That a before a person is not “to” - Spanish marks a human direct object that way (Lesson 21); a place or thing takes none.

Forms & patterns

The three main “boot” patterns:

Irregular yo (regular in every other person):

Examples

Common mistakes

• nosotros puedemos -> podemos - nosotros and vosotros keep the plain stem; the change happens only inside the “boot”.
• yo conozo -> conozco - these verbs take -zco, not -zo, in the yo form.
• yo sabo -> sé - saber has an irregular yo: sé (with an accent).
• yo teno -> tengo - tener adds -go to the yo form.
• conozco la respuesta -> sé la respuesta - facts and information use saber, not conocer.
• sé a tu hermano -> conozco a tu hermano - being acquainted with a person uses conocer.

Tips

• Picture the boot: the four changed forms surround the two unchanged ones (nosotros, vosotros).
• Dictionaries flag the change like poder (ue) or pedir (i) - trust that label.
• If the yo form ends in -go or -zco, the other five persons are usually regular.
• saber = a fact or a skill (sé la fecha, sé conducir); conocer = you have met/visited it (conozco a Ana, conozco Roma).
• The -zco/-go yo stem returns later in commands and the subjunctive (Lesson 38), so it is worth over-learning now.