1. must 侧重于个人意志和主观上的必要。have to 侧重于客观上的必要，可用于现在时、过去时和将来时。例如：
I know I must study hard. 我知道我必须努力学习。
My brother was very ill, so I had to call the doctor in the mid-night.
我弟弟病得厉害， 我只得半夜里把医生请来。
I haven't got any money with me, so I'll have to borrow some from my friend.
我身上没带钱，只好向朋友借点了。
He said they must work hard.他说他们必须努力工作。
2. have to可以用于多种时态；而must只用于一般现在或将来。例如：
The composition is due to hand in this morning, so I had to finish it last night.
作文今天早晨到期，因此我不得不昨天晚上完成。