突然这是一个不能说的秘密，英文怎么说啊？

This is not a secret. / This is a secret.

秘密本来就是不能说的..........
所以.....语法错了

suddenly,this is a secret can not be spoke out

Out of a sudden, this becomes an unspeakable secret.

This is a secret that can not be spoken out!~

This is a secret which can't to be spoke

This is a secret which shouldn't be revealed.