（not be）可以写成isn't吗

绝对不可以。
因为isn't 是is（be) not
所以，你弄反了。

not be是不是is not
还得具体看情况如果是单数就是isn't如果是复数就是aren't
过去式如果是单数就是wasn't 如果是复数就是weren't

is not才可以写成isn't

is not = isn't

至于not be 要看具体情况