It's+adj.+that 一般it的作用是形式主語(yǔ),也就是說(shuō)真正的主語(yǔ)是that引導(dǎo)的那個(gè)句子.例如：it's important that you helped me.
同位語(yǔ)從句是句式的一種,在復(fù)合句中充當(dāng)同位語(yǔ)的名詞性從句稱為同位語(yǔ)從句.一個(gè)名詞(或其它形式)對(duì)另一個(gè)名詞或代詞進(jìn)行修飾,限定或說(shuō)明,這個(gè)名詞(或其它形式)就是同位語(yǔ).例如：Last night we heared the news that our team had won.
主語(yǔ)補(bǔ)足語(yǔ)就是在被動(dòng)語(yǔ)態(tài)中,將賓語(yǔ)充當(dāng)補(bǔ)充主語(yǔ)的作用.例如：They caught boy stealing.被動(dòng)語(yǔ)態(tài) The boy was caught stealing.stealing 即為主語(yǔ)補(bǔ)足語(yǔ)