Radical changing

Question 1

How do radical changing verbs change when they contain an o (e.g. almorzar)?

Answer 1

o -> ue (e.g. almuerzo)

Question 2

How do radical changing verbs change when they contain an e (e.g. pensar; pedir), and which factor decides how they change?

Answer 2

e -> ie (e.g. pienso) for ar/er verbs

e -> i (e.g. pido) for ir verbs

Question 3

In which persons does radical changing occur?

Answer 3

all singulars (yo, tú, él/ella/usted), and third person plural (ell@s/ustedes)