Definition
Indicates that there was a period in the past when something was true or used to happen.
往昔；以前
Time was when this street was full of small shops.
曾经有段时间这条街上都是小商店。
Time was when people used to write letters to stay in touch.
曾经有段时间人们会写信来保持联系。
Time was when we would often picnic by the lake in the summer.
曾经有段时间我们夏天经常在湖边野餐。