it be + n. that 一般来说都是主语从句,因为同位语从句一般表示与之同位的名词(短语)的实际内容,是对前面的名词(短语)加以补充说明或进一步解释,而在主语从句中, it只是形式主语。指代的是that引导的从句。就拿it is a fact that xxxxxx来说,它表达的涵义是that xxxxxx is a fact,在句中充当主语,而that引导的同位语从句表达的是……样的fact,比如everyone surprises at the fact that……或the fact that…… surprises everyone希望帮得上你...