可能发生的事情，用正常时态e.g It looks as if there′s a storm coming.
与事实相反，用虚拟语气（往前退一个时态）e.g.
It seems as if he knew everything.
It looked as though they had lived in a pigsty.
because
主语不能是reason或cause
since或as不能引导表语从句
易混句式
原因
That/This is because…
The reason (why…)is that…
结果
That/This is why…
表语从句
用法
系动词后
“变”become/go/grow/get/turn
“不变”keep/stay/remain
feel/look/smell/taste/sound
seem/appear
prove/turn out
be动词
特殊引导词
as if/as though(好像）
可能发生的事情，用正常时态e.g It looks as if there′s a storm coming.
与事实相反，用虚拟语气（往前退一个时态）e.g.
It seems as if he knew everything.
It looked as though they had lived in a pigsty.
