Grammar: Radical Changing Verbs

Question 1

Pensar (to think)

Answer 1

Pienso (I think, am thinking)
Piensas (You think, are thinking)
Piensa (He/she/it/you polite sing. thinks, is/are thinking)
Pensamos (We think, are thinking)
Pensáis (You think, are thinking)
Piensan (They/you polite pl. think, are thinking)

Question 2

Other E-IE verbs:

Answer 2

Cerrar (to close) - cierro
Empezar (to begin) - empiezo
Merendar (to have a snack/picnic) - meriendo
Encender (to switch on) - enciendo
Entender (to understand, hear) - entiendo
Perder (to lose) - pierdo
Querer (to wish, want) - quiero
Sentir (to feel) - siento

Question 3

Volver (to return)

Answer 3

Vuelvo (I return, am returning)
Vuelves (you return, are returning)
Vuelve (he/she/it/you polite sing. return, is/are returning
Volvemos (We return, are returning)
Volvéis (you polite return, are returning)
Vuelven (they, you polite pl. return, are returning)

Question 4

Other O-UE verbs:

Answer 4

Almorzar (to have lunch) - almuerzo
Contar (to count, tell) - cuento
Encontrar (to find) - encuentro 
Probar (to try) - pruebo
Poder (to be able, can) - puedo
Morir (to die) - muero
Dormir (to sleep) - duermo

Question 5

Pedir (to ask for)

Answer 5

Pido (I ask for, am asking for)
Pides (you ask for, are asking for)
Pide (he/she/it/you ask for, is/are asking for)
Pedimos (we ask for, are asking for)
Pedís (you ask for, are asking for)
Piden (they, you ask for, are asking for)

Question 6

Other E-I verbs:

Answer 6

Medir (to measure) - mido
Repetir (to repeat) - repito
Seguir* (to follow) - sigo* (sigues, sigue etc.)
Servir (to serve) - sirvo