比较have to和must
1) 两词都是'必须'的意思，have to 表示客观的需要， must 表示说话人主观上的看法，既主观上的必要。
My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)
He said that they must work hard. 他说他们必须努力工作。（主观上要做这件事）
2) have to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。
He had to look after his sister yesterday.
3） 在否定结构中： don't have to表示"不必" mustn't表示"禁止"，
You don't have to tell him about it. 你不一定要把此事告诉他。
You mustn't tell him about it. 你一定不要把这件事告诉他。
have to和must是考试的'重要考点，所以同学们你们都懂了吗？说如果不懂得话，要记下来哦！
for与since 的辨析
for与since都能与现在完成时连用，但区别较大：
for后须接“时间段”，since后须接“时间点”，二者可以转换，均须与延续性动词连用 八年级，也可以与非延续性动词的否定式连用。二者引出的时间状语往往用 how long提问。如：
He has been here for two years．＝He has been here since two years ago．他来这里已有两年了。
—How long have you had this book？这本书你买多久了？
—For a week．／Since a week ago．买一周了。
We haven't heard from him for a month．我们已有一个月没收到他的来信了。
since能引导时间状语从句（主句用现在完成时，从句用一般过去时），而for则不能。如：
He has taught here since he came to China．自从他来到中国就在这儿教书。 It is ／has been an hour since he left here．他离开这儿已有一小时了。
学识谷
