23)
I think "we must show him he is wrong" would be better if you mean "he says or does something we think it's wrong, so we must prove that and make him realize".
24)
"We should ask him what he thinks" would sound better.
In fact "ought to" and "should" have the exact same meaning, which is to give advice. But "ought to" is a lot more formal and less used in modern English (which means it is quite old fashioned). So don't use it unless you're doing formal writings.
25)
The sentence is correct if you mean you are studying another subjects in college now (a fact) and you are regretting it, which means studying architecture is just your imagination. It has zero possibility, so using past perfect tense is correct.
There is an alternative: if you want to use a Type 3 conditional sentence, it might be "If I had made a choice, I would have studied architecture in college" if you mean you didn't have a chance to choose back then.
26)
It's correct if you want to tell someone you don't know how to play an instrument when they ask you to do so.
27)
It's correct. "Have to" means need to or is forced to do something, and it's in past tense because the incident happened last week.
