be against当中的against是介词，为什么前面还是be动词？

因为介词不能单独作谓语，故与be一起构成系表结构！

因为
against
后还应该带有宾语构成介词短语。介词短语可以充当状语、后置定语和表语。be
against
...
就是用介词短语。而如果要用介词短语的话，前面必须加上be动词。
例如：
It
is
against
our
policy.
这和我们的方针相抵触。
This
is
against
the
law.
这是违法的。
They
are
disinclined
to
use
violence
because
it
is
against
their
Buddhist
faith.
他们不愿意使用暴力，因为这么做违背他们的佛教信仰。
You
can't
do
anything
of
the
sort.
It
is
against
orders.
你不能干这事，这是违反命令的。
Judging
from
his
tone,
there's
no
doubt
he
is
against
it.
听他的话音，便知他是反对的。
Concealment
of
evidence
is
against
the
law.
隐藏证据是违法的。
against
英[əˈgenst]
美[əˈɡenst]
prep.
反对;
对…不利;
紧靠;
以防;
[例句]She
leaned
against
him.
她紧靠着他。

against
后还应该带有宾语构成介词短语，介词短语可以充当状语、后置定语和表语，be
against
...
就是用介词短语
“against
...”
作表语。

we
should
against
it
这句话对的
the
reasons
is
follow
这句话不对
首先
reasons是复数，要用
are
其次，
is
是谓语
follow
又是动词
，相冲突
改成
the
reasons
are
as
follow.
或者
the
followings
are
the
reasons.