by 和 till 均可用作介词，表示时间，意思是“到……时候”。具体区别如下：
1. by 的真正意思是“在……以前”、“到……时已经”、“最迟到……”等。如：
By now he will be eating dinner. 现在他一定是在吃饭。
I'll have finished this book by tomorrow. 到明天我就会把这书看完了。
I require my new coat by Monday. 我的新上衣要在星期一以前做好。
I'll finish the work by Saturday. 我将在星期六以前完成工作。
You must get the work done by Friday. 你必须最迟在星期五以前把工作做完。
You never know, it may stop raining by tomorrow morning. 说不定这雨到明天早上就不下了呢。
The outstanding debts must be paid by the end of the month. 未了的债务须在月底前偿还。
2. till 的意思是“到……为止”，其谓语动词应是持续性动词，若为终止性动词，则应为否定式。如：
He waited till Friday. 他一直等到星期五。
He didn't arrive till after midnight. 半夜过后他才到。
I won't qualify till next year. 我明年才具备资格。
I needn't leave till 9 tomorrow. 我不必在明天9点以前离开。
I needn't be leaving till 9 tomorrow. 我不必在明天9点以前离开。
We shan't know the results till next week. 我们下星期才能知道结果。