Stem-changing verbs
Click on the images below.  

Abrir = to open.  Abrir is a regular verb.  It does not change stem when you conjugate it.  Cerrar = to close.  Cerrar is a stem-changing verb.  Its stem changes from e -> ie when you conjugate it. 

Review of regular verbs:
Remember that to use a verb, or action word, in Spanish, you usually have to conjugate, or match, it to who's doing the action. There are 6 categories of who can be doing an action: 
  1. 1st person singular = I = yo = person speaking is doing the action
  2. 2nd person singular = you = tú / usted = person being spoken to is doing the action (usted is the more polite version of tú)
  3. 3rd person singular = he / she = él / ella = person or thing being spoken about is doing the action
  4. 1st person plural = we = nosotros / nosotras = person speaking is doing the action with one or more others
  5. 2nd person plural = y'all = vosotros / vosotras / ustedes = person being spoken to is doing the action with one or more others
  6. 3rd person plural = they = ellos / ellas = people or things being spoken about are doing the action
Therefore, there are 6 possible conjugations for verbs.  

Important!  Even though usted means you, verbs in the usted form will use the 3rd conjugation ending, same as he or she. Likewise, even though ustedes means y'all, verbs in the ustedes form will use the last conjugation ending, same as they.

Using regular verbs in Spanish is easy:
    1. Take off the -ar, -er, ir ending.  You're left with the stem of the verb.
    2. Add on the right ending, depending on who is doing the action.  
        a. For -ar verbs, your 6 options for endings are: o, as, a, amos, áis, an
        b. For -er verbs, your 6 options for endings are: o, es, e, emos, éis, en
        c. For -ir verbs, your 6 options for endings are: o, es, e, imos, ís, en

See the same information in a chart:
   -ar verbs: verb - ar    + o
 -er verbs:  verb - er   + o
 -ir verbs:  verb - ir   + o
See the conjugations of example verbs:
   hablar - ar = habl    + o
  comer - er  = com   + o
  vivir - ir  = viv   + o
See the verb conjugations' meanings in English:
 yo hablo 
 I talk 
 yo como 
 I eat 
yo vivo 
 I eat
 you talk 
 you eat 
 you live
 él / ella / usted habla 
 he / she talks / you talk 
 él / ella / usted come 
 he / she eats / you eat 
él / ella / usted vive 
 he / she lives / you live 
 nosotros / vosotras hablamos 
 we talk 
 nosotros / vosotras comemos 
 we eat 
nosotros / vosotras vivimos 
 we live
 vosotros / vosotras habláis 
 y'all talk 
vosotros / vosotras coméis 
 y'all eat
vosotros / vosotras vivís 
 y'all live
 ellos / ellas / ustedes hablan 
 they / y'all talk 
  ellos / ellas / ustedes comen 
 they / y'all eat 
  ellos / ellas / ustedes viven 
 they / y'all live 

Review of stem-changing verbs:
Stem-changing verbs have an extra step, if you choose any ending except for the 4th or 5th (nosotros or vosotros).  In this extra step, you have to make a small spelling change in the stem.
Stem-changing verbs will change in one of 3 ways:
    a. An o will change to ue (o-ue).
    b. An e will change to ie (e-ie).
    c. An e will change to i (e-i).

Let's take a look at the conjugations of example verbs. The ones below happen to all be -ir verbs. 
 sentir (e-ie) 
 dormir (o-ue) 
 servir (e-i)   
 yo siento 
 yo duermo
yo sirvo   
 él / ella / usted siente 
 él / ella / usted duerme
él / ella / usted sirve
 nosotros / vosotras sentimos 
 nosotros / vosotras dormimos
nosotros / vosotras servimos   
 vosotros / vosotras sentís
vosotros / vosotras dormís  
vosotros / vosotras servís   
 ellos / ellas / ustedes sienten
  ellos / ellas / ustedes duermen
  ellos / ellas / ustedes sirven

Remember that the only difference between stem-changing and regular verbs is this one spelling change.  Regular and stem-changing verbs follow the same conjugation rules and use the same verb endings. 

Stem-changing verbs don't come with ID tags.  So you have to memorize the lists of verbs that stem change.  In the previous lessons, you've studied the lists of the some of the most common stem changing verbs.   

Answer in-class questions:
  1. Mi hija vuelve a casa mañana.  ¿Quiénes vienen a tu casa este fin de semana?
  2. Yo entiendo un poco de italiano y portugués.  ¿Cuáles idiomas entiendes tú?