I'm sorry my assignment isn't finished. The thing is, I've had a lot of other work this week.

对不起，我的任务没完成。主要原因是我这星期有许多其他工作要做。

《牛津词典》

I'm sorry we can't come—we've got a lot on.

很抱歉我们去不了—我们安排得太满了。

《牛津词典》

I'm sorry to have troubled you a lot.

真抱歉让您费事了。

《新英汉大辞典》