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：this
is
a
book.
否定句：this
is
not
a
book
.
(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:he
eats
an
apple.
否定句:he
doesn't
eat
an
apple.
(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:i
teach
english.
否定句
i
don't
teach
english.
(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:i
can
dance.
否定句
i
can
not
dance.
或
i
can't
dance.
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: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：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：
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：
i
eat
some
apples.
否定句
i
don't
eat
any
apples.
(2)
见到and
变否定句时,要换成or.
例：there
is
some
air
and
water
on
the
moon.
否定句
there
isn't
any
air
or
water
on
the
moon.
(3)
见到too和also变否定句时,要换成either.
例:tom
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: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.