They always let me do what I should .这个句子为什么在should之后没加do?请高

They always let me do what i should这个句子should后应不应该加do? They always let me do what i think i should.这里为什么不用that而用wha They always let me do ________ I think I should.答案是what They always let me do ___ I think I should. a.that b.what They always let me do _____l think l should.为什么不能用that 必须用wh They always let me do______I think I should They always let me do______I think I should. what should i do?help me,plz What should I do? What should I do! what should i do what I should do?