1、I heared her singing in the next room just then.
聽(tīng)見(jiàn)時(shí)她正在唱歌.（hear sb. doing sth.聽(tīng)見(jiàn)某人正在做某事,hear sb. do sth.聽(tīng)見(jiàn)某人經(jīng)常做某事或做了某事）
2、just then“在那時(shí)”
I think it imporant to take exercise every day.
含有is為賓語(yǔ)從句,it為形式主語(yǔ)（不定式to take exercise 為真主語(yǔ)）；不含is為復(fù)合賓語(yǔ)（賓語(yǔ)+賓補(bǔ)）,it作形式賓語(yǔ)（不定式to take exercise 為真賓語(yǔ)）.