第一种情况：
要看句中
谓语动词是否有
系动词（is
/am/are/was/were）或者
情态动词(can/may/must/should/would/could/will/shall/need).以上两种情况只要符合其中之一，就在其系动词或者情态动词后直接加not即可(也可以用其缩略式)
其具体结构为：
主语+系动词/情态动词+not
/缩略式+V(原形)
+其他。
注意：如果句中含有并列词and时，变为否定句后要改为or。
例句：
1.The
twins
are
watching
TV
now.
The
twins
arenot(aren’t)
watching
TV
now.
2.They
were
in
the
front
of
the
classroom.
They
werenot(weren’t)
in
the
front
of
the
classroom.
3.The
child
can
play
the
piano
very
well.
The
child
cannot(can’t)play
the
piano
verywell.
4.We
students
should
help
each
other
in
study
.
We
students
shouldnot(shouldn’t)
help
each
other
in
study.
第二种情况：
如果句中谓语动词既无系动词又无情态动词而是
实义动词，则要用
助动词do或者它的其他形式does/did+not(或者用其缩略式don’t/doesn’t/didn’t）即可。
其具体的结构为：
主语+do/does/did+not+v原形+其他。
注意：如果句中含有并列词and时，变为否定句
后要改为or。
1.We
always
go
to
school
on
foot
every
day.
We
always
don’tgo
toschool
on
foot
every
day.
2.They
went
to
the
hospital
to
see
their
good
friend,
Tom.
They
didn’tgo
tothe
hospital
to
see
their
good
friend
,Tom.
3.My
good
friend
has
two
eggs
and
a
glass
of
milk
in
the
morning
beforehe
goes
to
school.
My
good
friend
doesn’thave
twoeggs
or
a
glass
of
milk
inthe
morning
before
he
doesn’tgo
toschool
.