yesterday 是什么时态

　　Yesterday's students are today's employees.

　　昨天还是学生，今天成了雇员。

　　I took a half day off yesterday.

　　昨天我休假半天。

　　That's the direct opposite of what you told me yesterday.

　　那与你昨天告诉我的截然相反。

　　Unless I'm mistaken, she was back at work yesterday.

　　除非是我记错了，她是昨天回来上班的。

　　They left the day before yesterday.

　　他们前天就走了。