今天我们将学习 "must"、 "should"、"have to"、"can" 和 "need to" 的否定句。
让我们先从包括 "must" 的否定句开始。
must
要构成使用 "must" 的否定句，只需要在 "must" 后加 "not" 即可。变成为：
must not
而 "must not" 的缩写形式是：
mustn't
"must not" 用来表示禁止一项动作或活动。即表示对某人表明不要做某事。看这些例子：
You mustn't walk on the grass.（你绝不能走在草地上）
He mustn't smoke in here.（你一定不能在这里抽烟）
We mustn't park in front of the entrance.（我们绝不能将车子停在入口前面）
You mustn't forget Jane's birthday.（你千万不能忘记珍妮的生日）
She mustn't lose that book, It's not hers.（她绝对不能遗失这本书。这不是她的）
We mustn't tell anybody. It's a secret.（我们绝不能告诉任何人。这是一个秘密）
You mustn't whistle in church. It's disrespectful.（你千万不能在教堂吹口哨。这是非常不敬的）
She mustn't talk with her mouth full. It's rude.（她嘴里塞满食物时绝不能说话。这是非常不礼貌的）
They mustn't play in the street. It's dangerous.（他们一定不能在街上玩。非常危险）
should
现在，我们来看包括 "should" 的否定句。"should" 否定句的形式，同样在 "should" 加 "not" 单词。
should not
它的缩写形式是：
shouldn't
它用来表示提供建议，并且不像 "mustn't" 那么绝对必要。例如：
You shouldn't walk on the grass.（你不应该走在草地上）
He shouldn't smoke in here.（他不应该在这里抽烟）
We shouldn't park in front of the entrance.（我们不应该将车子停在入口前面）
You shouldn't forget Jane's birthday.（你不应该忘记珍妮的生日）
有时需要针对你的建议给予一个理由。例如：
She shouldn't lose that book. It's not hers.（她不应该遗失这本书。这不是她的）
We shouldn't tell anybody. It's a secret.（我们不应该告诉任何人。这是一个秘密）
have to
我们来看看 "have to" 的否定句。"have to"动词的否定句，由添加 "do" 或 "does" 助动词所构成：
do not have to
并加上 "not" 到 "have to" 前面：
does not have to
缩写形式是：
don't have to
或者
doesn't have to
"Do not have to" 和 "does not have to" 用来表示不需要做某事。例如：
We don't have to worry. It's all right.（这你不必担心。一切都会没事）
I don't have to work on Saturday.（我周六不必工作）
He doesn't have to go by bus. He's got a car.（他不需要搭公车去。他有车）
She doesn't have to study. She knows it.（她不需要念书。她已经会了）
We don't have to wear to uniform.（我们不必穿制服）
You don't have to finish today. Do it tomorrow.（你们不必今天完成。明天再做）
They don't have to work. They are rich.（他们不需要工作。他们很有钱）
can
"can" 的否定句 是在 "can" 后加上 "not" 所构成：
cannot
它的缩写形式是：
can't
用来表示某事是不可能的。或某人不具有进行特定某事的能力。例如：
You can't walk on the grass.（你不应该走在草地上）
He can't smoke in here.（他不应该在这里抽烟）
We can't park in front of the entrance.（我们不应该将车子停在入口前面）
She can't treat me like this!（她不能这样对待我）
They can't sack you!（他们不能开除你）
We can't miss the wedding.（我们不能错过婚礼）
need to
最后，我们来看 "need to" 的否定句。"need to"的否定句先加上 "do" 或 "does" 助动词，随后放置 "not" 在 "need to" 的前面，变成为 ：
do not need to
或
does not need to
它的缩写形式是：
don't need to
或
doesn't need to
don't need to 和 doesn't need to 用来表示不需要做某事。与 "don't have to" 和 "doesn't have to"非常类似。例子：
You don't need to hurry. There's plenty of time.（你不需要着急。还有很多时间）
I don't need to pay until the end of the month.（我到月底前不需要支付）
He doesn't need to wash the car. it isn't dirty.（他不需要洗车。车不脏）
We don't need to take an umbrella. It isn't raining.（我们不需要带伞。没有下雨）
They don't need to wait outside. Tell them to come in.（他们不需要在外面等。叫他们进来）
——不断重复练习，才是学好英语的直径。