为什么“在那些日子”是in those days,而这些日子是these days,不是in these days

为什么“在那些日子”是in those days,而这些日子是these days,不是in these days 这些天是these days,那些天却是in those days,为什么?一个加in一个不加? these days和those days前要不要加in in these days是过去\将来\还是现在进行时? We are busy these days． In those days the workers had a hard in those days还是on those days that is the in thing these days there (be )few schools in these days has been ill in bed ____ three days .A.in these B.in those C The Smith family are ( )(在度假) in Beijing these days. in those days有没有在那些天的意思 in加一段时间用在将来时,那为什么in those days（在那些日子里）用在过去式.给我弄糊涂了?