虽然“until”和“by”这两个词都可以译成“到……时”，但它们的用法却不同。“Until”表示某事在未来的某个时间点前持续发生；“by”表示某事在未来某个时间或最后期限前发生。

1 Until

Until is used to talk about doing something up to a certain point in time.

“Until” 用来表示 “在某个时间点之前，一直做某事”。

I have to take care of my dog until 5pm.

Until the rain stops, we can't go to the beach.

I will be working until this evening. We can meet for dinner after that. 

2 By

By is used to talk about doing something before a future time or deadline.

“By” 用来表示 “在未来某个时间或最后期限之前做某事”。

I have to finish an essay by 5pm.

The council expects the bridge to be finished by December.

By the time I come back, I want this room clean.