比較have to和must
1) 兩詞都是'必須'的意思，have to 表示客觀的需要， must 表示說話人主觀上的看法，既主觀上的必要。
My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厲害，我只得半夜裡把醫生請來。(客觀上需要做這件事)
He said that they must work hard. 他說他們必須努力工作。（主觀上要做這件事）
2) have to有人稱、數、時態的變化，而must只有一種形式。但must 可用於間接引語中表示過去的必要或義務。
He had to look after his sister yesterday.
3） 在否定結構中： 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訴他。
You mustn't tell him about it. 你一定不要把這件事告訴他。
have to和must是考試的'重要考點，所以同學們你們都懂了嗎？說如果不懂得話，要記下來哦！
for與since 的辨析
for與since都能與現在完成時連用，但區別較大：
for後須接“時間段”，since後須接“時間點”，二者可以轉換，均須與延續性動詞連用 八年級，也可以與非延續性動詞的否定式連用。二者引出的時間狀語往往用 how long提問。如：
He has been here for two years．＝He has been here since two years ago．他來這裡已有兩年了。
—How long have you had this book？這本書你買多久了？
—For a week．／Since a week ago．買一週了。
We haven't heard from him for a month．我們已有一個月沒收到他的來信了。
since能引導時間狀語從句（主句用現在完成時，從句用一般過去時），而for則不能。如：
He has taught here since he came to China．自從他來到中國就在這兒教書。 It is ／has been an hour since he left here．他離開這兒已有一小時了。
