it is custom to take off the shoes 为什么用不定式呢?而不用that?如果要补充说明CUSTOM的话才用THAT引导从句吗?请看这个： it is almost certain that she will come

Question

it is custom to take off the shoes 为什么用不定式呢?而不用that?
如果要补充说明CUSTOM的话才用THAT引导从句吗?
请看这个： it is almost certain that she will come

Answer 1

不是不可以,只是句子要稍微有转换.
楼主的句子 "It is custom to take off the shoes",用that也可以,就换成:
It is custom that we take off the shoes.注意要加主语 we (或别的代词,如I),因为that引导的是从句,要主语.
第2句 It is almost certain that she will come.也可以转换成不定式,但同样要有改动,如下:
She is almost certain to come.注意这里改成 she 做主语了,不仅是句子的主语,也是 'to come' 的主语.
这样明白了吗?