on monday mornings还是in monday morning

.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。