1. 昨天
You use yesterday to refer to the day before today.
e.g. She left yesterday...
她昨天走了。
e.g. Yesterday she announced that she is quitting her job.
昨天她宣布打算辞职。
2. 过去；（尤指）不久前，近日，日前
You can refer to the past, especially the recent past, as yesterday .
e.g. The worker of today is different from the worker of yesterday.
如今的工人和以往的不同了。
e.g. ...a world without yesterdays or tomorrows.
没有过去和未来的世界