联系动词（连系动词）也称为系动词,后面一般接表语.
It seemed certain that their plane would crash.
it是形式主语,真正主语是从句that their plane would crash
本结构是
it+seem/be+adj +that-从句
It seemed certain that this would win theprize.
这会得奖,这似乎是肯定的了.their plane seemes to crash certainly可以吗不可以，意思不对。飞机似乎肯定坠毁。结构还是正确的。