1.On the bus I saw a student ____ I thought was your brother.
____ I thought was your brother是定语从句,修饰a student,i thought 是插入语,不影响成分,所以WHO was your brother,who 在从句中作主语.（ who/whom区别是看在从句中的成分,主语WHO,宾语WHOM（口语WHO））.
2.We were deeply impressed by the teachers and schools _____ we had visited.
同意 你的观点
作宾语 that或者不填 作had visited 宾语.
3.This is the very girl ____ I'm looking for.
A.whom B./
That.因为英文语法规定 the very ,the only修饰的先行词后面的关系代词用 that.例如：
(1) This is the very dictionary that I want to buy,
(2) After the fire in his house,the old car is the only thing that he owned.
此规则 人和物都适用!