.on Monday mornings，在星期一上午。
morning用复数形式是表示经常性的情况，它前面有修饰词Monday，所以介词要用on。.One Monday, there were fewer people in the shop than usual when the woman came in, so it was easier for the detective to watch her.有一个星期一，当这位妇女走进这家商场时，里面的人比往常少，因此，侦探比较容易监视她。
温馨提示：
