一般现在时变否定句的规则.
(a) 含有 am is are 的句子,变否定句时,
只需在am is are 后加not.
例1 This is a book.
否定句 This is not a book .
例2 I am a teacher .
否定句I am not a teacher .
例3 These are pens .
否定句These are not pens .
(b) 含有实义动词的句子,变否定句分两步.
（1）如果主语是第三人称单数时,要在实义动词前加doesn't,后面的动词用原形.
例1 He eats an apple.
否定句 He doesn't eat an apple.
例2 Jim does his homework.
否定句 Jim doesn't do his homework.
例3 Marry goes to school.
否定句Marry doesn't go to school.
(2) 如果主语不是第三人称单数时,只 需在实义动词前加don't.
例1 I teach English.
否定句 I don't teach English.
例2 They go to school.
否定句 They don't go to school.
例3 We sing a song.
否定句 We don't sing a song.
(c) 含有情态动词的句子,变否定句时,只需在情态动词后加not.
例1 I can dance.
否定句 I can not dance.
或 I can't dance.
例2 They can speak English.
否定句 They can not speak English.
或 They can't speak English.
例3 He must go home.
否定句 He must not go home.
或 He mustn't go home.
例4 He must be on the playground.
He can not be on the playground.
析：当must be用来表示推测,有“一定”的含义时,
否定式要用can not.
例5 You must say something at the mecting.
You needn’t say anything at the mecting.
析：当“must+行为动词”用来表示“必须”的含义时,
其否定式可用needn’t 或mustn't表示“不需要”,“不必”,
语气委婉,must not表示“禁止”,“千万别”语气生硬.
例6 You may come in now.
You mustn't come in now.
析：may作为情态动词表示“可以”时,
其否定式用must not,而不用may not.
例7 He may watch TV .
He mustn't watch TV.
注意:
(1) 见到some 变否定句时,要换成any.
例1 I eat some apples.
否定句 I don't eat any apples.
例2 He has some books.
否定句 He doesn't have any books.
例3 He does his home work.
否定句 He doesn't do his home work.
(2) 见到and 变否定句时,要换成or.
例1 There is some air and water on the moon.
否定句 There isn't any air or water on the moon.
例2 He has some brothers and sisters.
否定句 He hasn't any brothers or sisters.
例3 He eats apples and oranges.
否定句He doesn’t eat apples or oranges.
(3) 见到too和also变否定句时,要换成either.
例1Tom goes to school too.
否定句 Tom doesn’t go to school either.
too和also用于肯定句中时,否定句中用either替换.
（4 见到already变否定句时,要换成yet.
例1 He has had supper already.
He hasn’t had supper yet.
随着对谓语动词的否定,某些副词也要作相应的变动.