I must finish doing my homework before today.
= I have to finish doing my homework before today.before today比如改成before lunch(那中间需加定冠词the吗?改为before the lunch 对嘛?）为什么不用加 the, before lunch可那是特指这天的中午之前完成作业，为什么不家呢?lunch 前一般不用the如： What do you have for lunch ?可那是特指这天的中午，为什么不家呢?开学可以问问你的英语老师哟