看到了如此两句：
Houses are a lot more expensive these days. 现在的房价贵多了。
The children of the poor couldn’t go to school in those days. 那时没钱人的小孩上不了学。
上面两句中，these days 前没用介词 in，但 those days 前就用了介词 in。这两个短语前的介词用和省略有哪些特征和规律吗？
■陈根花老师在“英语语法网”有一篇非常不错的文章，可以解决你的问题。
《that day, this day, these days, those days前用介词吗》
……
■these days 的意思是“近来”“现在”“目前”，其前习惯上不加介词。如：
I rarely see her these days. 近来我极少见到她。
A pound doesn’t go far these days. 如今一英镑不经一用。
News travels quickly these days. 近来消息传播得非常快。
Jobs are hard to come by these days. 近来，非常难找到工作。
I don’t have much time to read these days. 我这类日子没什么时间念书。
■those days 前一般要用介词 in，短语 in those days 的意思是“那时”“当时”“那年头”“那年月”等，in those days 在很多状况下与 at that time 意思相同。如：
We were poor in those days. 那时大家非常穷。
Life was hard in those days. 在那些日子里，生活非常苦。
I was still living with my parents in those days. 我那时还跟爸爸妈妈住在一块。
only a small number of people went abroad in those days. 那时只有少数人出国。
It wasn’t quite the done thing for women to drink in pubs in those days. 那年月女的到酒店喝酒可不成体统。
注意：these days 与 in those days 虽然形式相近，但其中的一个习惯上要用介词，而另一个则习惯上不需要介词，注意不要搞混淆了。
■原文见：http://www.yygrammar.com/Article/201011/1809.html
