Hello! 
1. yo / almorzar
The verb "almorzar" means to eat/have lunch. It also is a stem-changing verb. This means that the "o" changes to "ue" <u>only</u> when the subject is yo, tú, él, ella, ellos or ellas. This never occurs in the nosotros/vosotros subject. 
Answer: Yo / alm<u>ue</u>rzo 
2. tú / cerrar 
The verb "cerrar" means to close, and it is also a stem-changing verb. In this case, the "e" changes to "ie". Since the subject is "tú", the stem-change occurs. 
Answer: Tú / c<u>ie</u>rras
3. nosotros / pensar 
The verb "pensar" means to think, and is also a stem-changing verb. The "e" changes to "ie", but the subject is nosotros, so there is no stem-change. 
Answer: Nosotros / pensamos 
4. ellos / dormir 
The verb "dormir" means to sleep, and it is also a stem-changing verb the like the other. The "o" is changed to "ue". Since it does not have the subject nosotros or vosotros, the stem-change occurs. 
Answer: Ellos / duermen 
5. yo / traer 
The verb "traer" means to bring. This verb is interesting because it is a -go verb. This means that when the subject is yo, the verb ends in -go, and it also replaces the "e" with an "i". 
Answer: Yo / tra<u>ig</u>o 
6. nosotros / decir 
The verb "decir" means to say/tell. It is also a -go verb, so the ending is replaced with -go when the subject is yo. Since the subject is nosotros, there is no special change, except the ending is changed due to the subject. 
Answer: nosotros / decimos 
7. él / volver 
The verb volver means to return, and it is also a stem-changing verb. Like "dormir", the "o" is changed to "ue" when the subject is yo, tú, él, ella, ellos, or ellas. 
Answer: Él / v<u>ue</u>lve 
8. yo / venir
The verb "venir" means to come, and is also a -go verb in the subject yo. 
Answer: Yo / ven<u>go</u> 
9. ella / perder 
The verb "perder" means to lose, and is a stem-changing verb. The "e" is replaced with "ie" when the subject is yo, tú, él, ella, ellos, or ellas. 
Answer: ella / p<u>ie</u>rde
10. tú / entender 
The verb "entender" means to understand, and is a stem-changing verb. For this verb, the "e" is replaced with "ie", and since the subject is tú, the stem-change occurs. 
Answer: Tú / ent<u>ie</u>ndes
<u>Final answers</u>: 
- Yo almuerzo
 - Tú cierras
 - Nosotros pensamos 
 - Ellos duermen 
 - Yo traigo 
 - Nosotros decimos 
 - Él vuelve 
 - Yo vengo 
 - Ella pierde
 - Tú entiendes