By 字的運用

by this weekend = 今個星期六或之前
by this weekend = not later than this weekend 不遲於本週末
e.g.
Hand in your work by Friday => Friday is the last day to hand in your work.

by a certain time in the future = 到那時候
e.g.
Can you finish the work by 5 o'clock? 你可以在五點前完成嗎﹖
By this time next week, they will be in New York.

by a certain time in the future 動詞的時態多為 future perfect tense
e.g.
I will have read the book once by this time tomorrow. 明天這時候我會(已經)把書讀完一遍。
He ought to have arrived by now (by this time). 這時候他應該已經抵逹。
By the time the postcard reaches you, I will have left the country. 你收到明信片時，我已經離開國家。




2009-01-14 08:47:03 補充：
You can use either
Hand in your work before Friday or Hand in your work by Friday
but the deadline is different.

2009-01-14 08:48:13 補充：
You cannot use before in this sentence:
By this time next week, they will be in New York.
Before this time next week, they will be in New York. (wrong)