為什麼when - 後面要用非謂語
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when 後面本來有主語和 be 動詞,還有動詞 ing,有時be動詞和動詞ing可以省略,這樣就只剩下了 動詞ing。
例句
When I was doing my homework last night, I heard someone knocking at the door. 從句和主句的主語是相同的一個人,這時上句可以改寫成
When doing my homework last night, I heard someone knocking at the door.