we belong together forever.

We belong together, forever.

He is with Mary together, forever.

Forever = adwerb

We are together forever
We belong to each other forever
这样都还比较对

He and mary are together forever
这样

forever 副词 修饰的是together ‘永远在一起’

这两就都是Chinglish

应该说

We belong to each other forever.

He will be with Mary forever.

我们永远属于彼此：We belong to each other forever. 或 We are together forever.
第二句：He is together with Mary forever. together 是副词，修饰介词词组be with

修饰前面we belong together..
we belong together本来就可以成单独的句话