“Fuimos” is this answer and no is “exto” is “sexto”
It is "va" because the a in va represents he, she, and you (formal). He is going to the pool, so it is 'Mi padre va a la piscina con nosotros.'
1: A stem change verb is a verb that also makes the change in the stem
2: I'm not sure what this one is i'm sorry
3: -empezar is e-ie
-poder is o-ue
- pedir is e-i
4: i'm very sorry I don't know this one either... I'm just looking at a paper I got in my spanish class for all of the answers
5: there are four types of stem-changing verbs
>o-ue
>e-ie
>e-i
>u-ue