Stem-changing verbs change more than just the endings when you conjugate them. The vowel in the stem (the part of the verb left when you remove the -ar, -er and -ir endings) will change in these particular verbs from a single vowel to a double vowel, or from one vowel to another.
However, these verbs do not stem-change in the nosotros form (Think "no!" for NOsotros!) This is why stem-changing verbs often are refered to as "Boot" or "Shoe" verbs. When we draw a box around the stem-changed conjugations, we duck under Nosotros (because it doesn't change) which makes a boot-type shape. When stem-changing, NOsotros is NOrmal!
The first category is the e-->ie stem-changers. We have seen one example with the verb Tener.
Verbs that change like Tener are called e-->ie stem-changers because the "e" in the stem changes to "ie".
|Gumby tiene un caballo.||Gumby has one horse.|
|Nosotros tenemos tres perros y un caballo.||We have three dogs and a horse.|
The second category is the o--> ue stem-changers. A good example of this type of stem-changer is Almorzar.
|Almuerzo con mis amigos.||I eat lunch with my friends.|
|Almorzamos en la cafetería.||We eat lunch in the cafeteria.|
Dormir and Poder are two other examples.
Included in this category is the unique verb Jugar. It falls under the O-->ue stem-changer classification since the verb's original form was Jogar.
|Vershara juega al golf.||Vershara plays golf.|
|Jugamos los sábados.||We play on Saturdays.|
The last category is the e-->i stem-changers, like Pedir . This e-->i change occurs only in "Ir" verbs
|Pido un bolígrafo.||I ask for a pen.|
|Pedimos más café.||We ask for more coffee.|
Other examples are Decir [to say or tell], Repetir [to Repeat], and Servir [to serve].
Which vowel changes in a stem-changing verb?
First we want to start the conjugation process as we normally would: we remove the last two letters (-Ar, -Er or -Ir.) Then, if the verb is a stem-changer, the very next vowel to the left will be the one that changes:
Entender -----> Entend (-er) ---> Entend- ---> Entiend-
Preferir ------> Prefer (-ir) ----> Prefer- ----> Prefier-
You can use this rule to identify verbs which cannot be stem-changers since only the vowels "E" and "O" can stem-change:
Decidir (the next vowel after the ending is removed is an "i" which cannot change)
Acampar (the next vowel after the ending is removed is an "a" which cannot change)
Buscar (the next vowel after the ending is removed is a "u" which cannot change)