他们都认为它们非常时尚.they both think them are very fashion.

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有both不能放这里 them是宾格不能做主语 fashion名词不能做表语
正确：They think both of them are very fashionable.
======以下答案可供参考======
供参考答案1:
they both think themselves are very fashionable.
供参考答案2:
fashionable
供参考答案3:
They think that both of them are very fashionable
fashion是名词不能这样用，them是 宾语，they才是主语，不能这样用