be动词后面的动词形式有三种可能：
①-ing:
现在进行时,I'm
trying
to
give
you
the
right
answer.
②-ed:
被动语态,I'm
told
that
you
don't
know
her.
③-to
do:
1、表示按计划或安排要做的事.例如：They
said
goodbye,little
knowing
that
they
were
never
to
meet
again.他们告了别,不知道以后再也不会见面了。
2.表示“应该”,相当于should,ought
to.例如：You
are
to
report
to
the
police.你应该报警。
3.表示“必须”,相当于must,have
to.例如：The
letter
is
to
be
handed
to
him
in
person.这封信必须亲手交给他。
4.表示“想,打算”,相当于intend,want.例如：If
we
are
to
be
there
before
ten,we’ll
have
to
go
now.如果我们要在十点前到,我们现在就得走。