I have three questions to ask someone.Come in for help me.

18. We hear you _went_______ to Udorn three days ago.

why the answer isn't have gone??
We use went but not have gone because the action (go) is already passed, so we use a simple past tense (went) here.
10. - Are you going to visit him? - If she goes, __so will I______ .
(A) I do so
(B) so will I
(C) so I go
(D) so I do

(A) Are you going to visit him? – If she goes, I do so. – wrong because this sentence is missing the word will. This is a conditional sentence, that is, if you xxxx, I will xxxx.

My grandmother used to walk 5 miles to go to church on Sundays

why the answer isn't is used to walking??
Used to = phrasal verb = verb + preposition = a special meaning.
Used to = always do.

three days ago is [we heard not we hear, and use went to....]

are you going to visit him? if she goes, so will i
你是否去探訪他? 假如她去, 所以我也會去。[是因為她去
你才會去, 若然她不去, 你是不會去的。用will i 即是 ~ 我也會 ~的意思]

to walk 是指每星期日每做的事