英语中动词不定式在什么情况下使用

动词不定式是由“不定式符号to+动词原形”构成的一种非谓语动词结构。有些动词不定式不带to，动词不定式可以作句子的主语、表语、宾语、定语、补语、状语或单独使用。不定式保留动词的某些特性，可以有自己的宾语、状语等。动词不定式和它后面的宾语、状语等一起构成短语，叫作不定式短语。现对九年级动词不定式的用法作一次简要梳理。
　　一、用作主语
　　直接把动词不定式置于句首的情况不多，多数情况用it作形式主语，把真正的主语——动词不定式置于句末，特别是不定式短语较长时。动词不定式作主语，谓语动词用第三人称单数形式。带疑问词的不定式短语作主语常置于句首，复合不定式结构作主语常置于句末。如：
　　1.As
young
adults,
it
is
our
duty
to
try
our
best
to
deal
with
each
challenge...
(p.8)
　　2.The
head
teacher
said
it
was
necessary
to
talk
with
his
mother.
(p.16)
　　3.How
to
make
requests
politely
is
important.(p.93)
　　二、用作表语
　　动词不定式作表语，常说明主语的内容、性质、特征。如：
　　1.The
best
way
to
improve
your
English
is
to
join
an
English
club.
(p.7)
　　2.The
first
thing
is
to
greet
the
teacher.
(p.96)
　　三、用作宾语
　　1.可以接带to的动词不定式作宾语的动词主要有：要求选择同意(ask,
choose,
agree)，期望决定学习(expect,
hope,
decide,
learn)，宁可假装知道(prefer,
pretend,
know)，希望想要愿意(wish,
want,
would
like
/
love)。如：
　　1)We
decided
to
talk
to
some
students
about
why
they
go
there.
(p.88)
　　2)He
prefers
to
eat
white
bread
and
rice.(p.50)
　　3)Id
love
to
visit
Mexico.(p.53)
　　2.动词decide,
know,
learn,
show,
teach,
tell...，介词结构on...可用疑问词带to的不定式短语作宾语，但why后面的不定式不带to。如：
　　1)Could
you
please
tell
me
where
to
park
my
car?(p.92)
　　2)It
gives
advice
on
what
to
do
in
lots
of
different
situations.
(p.32)
　　3.动词feel,
find,
make,
think等后面，可以用it作形式宾语代替真正的宾语——动词不定式，句子结构是...feel
/
find
/
make
/
...
it+adj.
/
n.+to
do...。如：
　　I
find
it
difficult
to
remember
everything.
(p.98)
　　4.既可接动词不定式又可接ving形式作宾语，意思差别不太大的动词有begin,
start,
like,
love等。一般说来，动词不定式表一次、数次的具体动作、将来动作或动作的全过程，ving形式表习惯性的连续动作。但是这种区别并不很严格，特别是美国英语，即使是习惯性的动作也可以用动词不定式。如：
　　1)Then
I
started
to
watch
English-language
TV.(p.6)
　　2)I’m
beginning
to
understand
my
Chinese
roots,
and
who
I
am.
(p.116)
　　begin用了进行时态，后面只接不定式作宾语，表示事情刚开始，还要继续下去。
　　3)I
like
to
eat
vegetables.
(p.118)
　　感知动词like,
love,
know等一般不用进行时态。

1.用在一些及物动词后，如
want
to
do
fail
to
do
decide
to
do
manage
to
do
plan
to
do
2。用在一些疑问词后可做主语，宾语，表语等。When
to
start
is
not
decided.
3
用在介词but后做介词宾语，如
I
did
nothing
but
watch
TV
yesterday./
We
had
no
choice
but
to
wait
here.
4。用在动词后也可在句首做状语。如
He
came
to
see
me./
To/In
order
to
catch
the
early
bus
,he
got
up
early.
5。用在系动词后做表语。如My
dream
is
to
be
a
scientist.
6.
用在句首做主语,
To
be
a
singer
is
my
dream.
7.用在名词或代词后做后置定语，
The
best
time
to
come
to
Beijing
is
autume.
8.用在宾语后补充宾语做了什么，叫宾语补语
Our
teacher
tells
us
to
read
more
in
the
morning
.