1、the only long period of time during it the whole family could be together once a year is the Spring Festival.这句话的主语是the only long period of time
during which引导的是一个定语从句the whole family could be together once a year,which 是一个引导词
2、副词可以直接用.my parents were always busy
是一个系表结构,be是一个系动词
3、动词需要变成分词形式才能充当主语.
4、we should concentrate on ours.是tell的宾语从句,引导词用that,如果跟在tell之后可以省略,但前面有一个and为了避免歧义,that不能省略.