jacky
can与be
able
to表示能力的时候，是同义的。如：
He
can
speak
two
foreign
languages.
He
is
able
to
speak
two
foreign
languages.
它们用法上的不同主要体现在以下几方面：
⒈can只有两种时态，即can和could，而be
able
to有多种时态，如was/were
able
to，will/shall
be
able
to，have/has
been
able
to等：
I'll
be
able
to
drive
the
car
in
a
week.
Through
his
diligent
work，the
deaf-mute
has
been
able
to
read
and
write
in
a
sign
language.
⒉表示过去通过努力终于做成了某事要用be
able
to，而不能用can。如：
I
was
able
to
swim
to
the
bank
after
the
boat
turned
over.
can
和be
able
to
都可以表示能力。can泛指一般的能力，而且只有两种形式，即can/could。be
able
to则主要指具体做到了某件事的能力，其形式主要是靠be发生变化，所以形式比can多。
可以说：I
can
swim.
I
am
able
to
swim.
但是不能说：All
the
people
could
escape
from
the
big
fire
in
time.
只能说：All
the
people
were
able
to
escape
from
the
fire
in
time.
⒊can能表猜测，be
able
to不能。can的这种用法主要用于疑问句和否定句。
Somebody
is
knocking
at
the
door.
Who
can
it
be?
It
can't
be
our
teacher
who
is
knocking
at
the
door.
其肯定形式是must
be。如：
It
must
be
our
teacher
who
is
knocking
at
the
door.
must
have
done
something
表示对现在完成或过去动作的猜测，其否定形式是can't
have
done。如：
-His
parents
must
have
gone
abroad.
-No,
I
don't
think
so.
They
can't
have
gone,
for
I
saw
them
only
this
morning.
要特别注意它们的反意问句形式：
He
must
be
a
good
teacher,
mustn't/isn't
he?
The
students
must
have
gone
to
bed,
haven't
they?
They
must
have
finished
their
lessons
yesterday,
didn't
they?
could既可以用肯定形式，又可以用疑问或否定形式。如果用could表示现在“可能”，比can语气更加委婉。如：
If
you
don't
spend
enough
time
on
your
studies,
you
could
fail
in
the
final
examination.
It
could
be
our
teacher
who
turned
off
the
light
last
night.与It
must
be
our
teacher相比，这句话表示的可能性更小，因为它表示了两种可能性，即“可能是”也可能“不是”，而must
be只表示了一种猜测。
could
have
done这个句型有它的特殊含意，表示“本来可能做到而未做到的事”。如：
Our
class
team
could
have
won
the
game.
再看以下例句：
He
cannot
be
over
praised
for
what
he
has
done
for
all
of
us.可能有同学会理解为“不能因为他为我们大家做了这些事情而过分表扬。”其实这样理解是错的，因为这里的cannot
be不是表“不能”，而是表“不可能是”的意思，因此这句话的意思是“他为我们大家做了这些事，应该受到大大的表扬。”即“再怎么表扬也不过分”。
⒋can可以表“允许”，与may可以互换，此时不能用be
able
to代替。如：
-Can/May
I
sit
here?
-Yes，
please.”
由于may
not有两个意思，即“不可能”和“不许可”，因此使用时要特别注意。如：
He
may
not
go
home
this
weekend.此句既可以表示He
will
probably
not
go
home
this
weekend.也可以表示
I
don't
permit
him
to
go
home
this
weekend.在口语中，表达这两种不同的含义是通过句重音来解决的。如：
He
may
`not
go
home
this
weekend.
(I
don't
permit
him
to
go
home
this
weekend.)
He
`may
not
go
home
this
weekend.
(He
will
probably
not
go
home
this
weekend.)
而在书面表达时不可能使用句重音的手段，所以最好用can't代替may
not来表示“不可能”：
He
can't
go
home
this
weekend.
⒌can是情态动词，只能用作谓语成分，be
able
to是普通动词短语，既可以作谓语，也可以作非谓语。如：
Being
able
to
speak
English
fluently,
he
has
no
difficulty
in
communicating
with
the
Americans
at
the
party.
How
I
long
to
be
able
to
communicate
with
the
foreigners
in
fluent
English!
⒍be
able
to
后边接动词不定式表示一种实际情况时，其否定形式不是be
not
able
to，而是cannot。如：
-Were
you
able
to
catch
the
first
bus
yesterday
morning?
-No，I
couldn't.
⒎一般说来，be
able
to后边的动词不定式没有被动语态。如:
I'm
sure
he
is
able
to
correct
his
mistakes
in
grammar.
不说：I'm
sure
his
mistakes
are
able
to
be
corrected
by
him.
但是美国近来也出现了Being
able
to
be
hurt
by
things这种结构，美国专家解释为English
is
becoming
looser.
(英语变得越来越不严谨了)，大家不宜模仿。
