1、I heared her singing in the next room just then.
听见时她正在唱歌.（hear sb. doing sth.听见某人正在做某事,hear sb. do sth.听见某人经常做某事或做了某事）
2、just then“在那时”
I think it imporant to take exercise every day.
含有is为宾语从句,it为形式主语（不定式to take exercise 为真主语）；不含is为复合宾语（宾语+宾补）,it作形式宾语（不定式to take exercise 为真宾语）.