by the end of next week

下星期前完結 (before the end of next week)

i want the proposal by the end of next week
i want the proposal before the end of the week

homework is due the end of next week
you must hand in the homework by the end of next week
you must hand in the homework before the end of next week

下星期完結前

下星期完結前
I will finish the work by the end of next week.

下星期完結前

by: prep., 不遲於;在...之前

e.g.1 By the end of next week, we have to hand in this project.
在下星期完結前，我們要完成這份專題報告

e.g. 2 We had to get there by evening.
我們必須在夜晚前抵達那裡。