我们都知道，must 的过去形式为 had to, 意思是“必须，不得不”，如：I must go to school now. 我现在必须去上学。
但如果是过去时态就应该是：I had to go to school yesterday. 我昨天必须去上学。这句话中就不能用must.
但是，must 也有用于过去时态的情况，今天我们一起来学习一下。
例句1：Last night I told my parents that I must finish my homework before 9:00 pm.
昨天晚上我告诉父亲我必须在晚上9点前完成作业。
例句2：Yesterday morning I considered how I should spend the day. First I must finish my homework.
昨天早上我考虑如何度过这一天。首先我必须完成我的家庭作业。
例句3：There were some tasks that I must finish first.
有一些任务我必须得先做完。
例句4：As I was talking on the phone with my parents, someone must knock at the door.
当我正在和父母通电话时，偏偏有人来敲门。
