关于虚拟语气的疑问：虚拟语气有主从句之分,让我很困惑的不含if的虚拟语气.

as if / though 引导的虚拟语气,虚拟只是在从句中使用,主句中不用虚拟, 与现在事实相反用过去时,与过去事实相反用过去完成时.如果是所述的是真实的,就用直陈语气.
eg ,It sounds as if it is raining． 听起来像是在下雨. （直陈语气）
You look as if you didn’t care． 你看上去好像并不在乎 （ 与现在事实相反）
He talks about Rome as if he had been there before． 他说起罗马来好像他以前去过罗马似的（与过去事实相反的虚拟语气.
If 条件句中的虚拟要分主句和从句,
从句与将来,现在和过去事实相反用
If 主语+ were to do/were / did / had done
而 主句 表示与将来,现在和过去事实相反,主句应该是
主语 + would /could/might + do /have done .
eg .If I were you , I would not do it .
If he had driven carefully enough , the accident would not have happened.
If only 是从句, 虚拟就和If 条件句相同.
If only hadn't done that !