其实很简单，但首先要从动词开始说起。只有“实意动词”才会涉及到您说的问题，也就是“
主谓一致原则”。句子分为“肯定句”、“否定句”和“
疑问句”三种类型。在“肯定句”中，
谓语动词要与主语保持一致：当主语为“
三单（第三人称单数的简称）”时，谓语动词要采用“三单”形式，要在词尾加-s或-es，具体变法在此略去。举几个例子吧：
1)
Li
Weikang
goes
to
school
by
bike
every
day.
2)
Lucy
has
lunch
at
school.
3)
Tom
wears
yellow
T-shit
and
blue
shorts.
4)
He
studies
hard
at
school.
5)
She
comes
to
see
me
once
a
week.
当句子为“否定句”或“疑问句”时，需要加“
助动词”来构成“否定”和“疑问”，但此时助动词后面的动词要用“原形”。还以上面的几个句子为例，分别变成“否定句”和“疑问句”，并作回答。
1)
Li
Weikang
doesn't
go
to
school
by
bike
every
day.
（否定句）
Does
Li
Weikang
go
to
school
by
bike
every
day?
(疑问句）
Yes,
he
does.
/
No,
he
doesn't.
（回答）
2)
Lucy
doesn't
have
lunch
at
school.
（否定句）
Does
Lucy
have
lunch
at
school?
（疑问句）
Yes,
she
does.
/
No,
she
doesn't.
（回答）
3)
Tom
doesn't
wear
yellow
T-shirt
and
blue
shorts.
（否定句）
Does
Tom
wear
yellow
T-shirt
and
blue
shorts?
（疑问句）
Yes,
he
does.
/
No,
he
doesn't.
（回答）
4)
He
doesn't
study
hard
at
school.
（否定句）
Does
he
study
hard
at
school?
（疑问句）
Yes,
he
does.
/
No,
he
doesn't.
（回答）
5)
She
doesn't
come
to
see
me
once
a
week.
（否定句）
Does
she
come
to
see
me
once
a
week?
（疑问句）
Yes,
she
does.
/
No,
she
doesn't.
（回答）
当然了，当主语为“非三单”时，若变“疑问句”和“否定句”也要加“助动词”的，举一个例子：“他们放学后在操场上踢足球”：
They
play
soccer
on
the
playground
after
school.
（肯定句）
They
don't
play
soccer
on
the
playground
after
school.
（否定句）
Do
they
play
soccer
on
the
playground
after
school?
（疑问句）
Yes,
they
do.
/
No,
they
don't.
（回答）
现在你看明白了吗？我为了方便学生记忆，给他们编了两个“口诀”，做题时就不会出错了：
情态动词、助动词后，动词还原。（这指的是主语为“三单”时的用法）
用什么问，用什么答。（这指问句用的助动词与回答时所用的助动词，在“形”和“数”上保持一致，还有情态动词、there
be等也都是如此。）举例：
1)
There
is
a
big
tree
in
front
of
my
house.
（肯定句）
There
isn't
a
big
tree
in
front
of
my
house.
（否定句）
Is
there
a
big
tree
in
front
of
your
house?
（疑问句）
Yes,
there
is.
/
No,
there
isn't.
（回答）
（DAVID提供）