Vicente y francisco / jugar / al vóleibol los domingos adela y yo / empezar / a tomar clases de tenis ustedes / volver / de cancún el viernes los jugadores de béisbol / recordar / el partido importante la profesora / mostrar / las palabras del vocabulario adán / preferir / escalar la montaña de noche (yo) / entender / el plan de estudios (tú) / cerrar / los libros y te vas a dormir

We will use this exercise using the concept of Spanish Present Tense Forms. In Spanish, the simple present tense (el presente or el presente de indicativo) is used to talk about habitual actions, routines, things happening now or in the near future, universal truths, facts, hypotheticals, lapses of time, and for ordering in restaurants and stores. There are both regular and irregular indicative forms. So, we have regular and irregular present indicative forms. 

In Spanish, you can recognize some endings for regular verbs, one for ar verbs, one for er verbs and one for ir verbs. Thus, if you want to conjugate an ar verb, take away the infinitive ending ar and add the ending that matches the subject. On the other hand, if you want to conjugate an er verb, remove the infinitive ending er and add the ending that matches the subject. Finally, to conjugate an ir verb, take away the infinitive ending ir and add the ending that matches the subject.

There are some changes when using the present tense yo conjugations, these changes build up the irregular form. In this way, verbs ending in guir, ger, or gir undergo a spelling change in their present tense yo forms. First, for verbs ending in guir, the yo form ends in go. Second, for verbs ending in ger or gir, the g in the yo form changes to a j. On the other hand, there are some exceptions regarding ar, er and ir verbs, so there are some of these types of verbs that have an irregular form.

Therefore, the solution of this problem is as follows:

1. Vicente y Francisco juegan al Voleibol los domingos. 

Jugar: Verb ending in ar

2. Adela y yo empezamos a tomar clases de tenis. 

Empezar: Verb ending in ar

3. Ustedes vuelven de Cancún el viernes 

Volver: Verb ending in er

4. Los jugadores de béisbol recuerdan el partido importante

Recordar: Verb ending in ar

5. La profesora muestra las palabras del vocabulario

Mostrar: Verb ending in ar

6. Adán prefiere escalar la montaña de noche

Preferir: Verb ending in ir

7. Entiendo el plan de estudios 

Entender: Verb ending in er

8. Tú cierras los libros y te vas a dormir

Cerrar: Verb ending in ar

Ithink los discos compactos.