as if引导从句的虚拟语气比较特别些；它的形式与主句时态并不呼应,而是由as if从句动作的情况所决定.
On arriving at the gate of the college,my heart started to beat fast as if it_______out of my mouth．
A．would leap B．had leaped C．should leap
这个句子明显是与过去相反的假设,所以,得用B
就好象它（当时）要跳出我的嘴巴似的.
He looks as if he were ill(好象现在生病）
He looks as if he had been ill(好象生过病）