I am a little confused in using (It is) in these kinds of sentences:
1) You're the only one who believes that it is true

, or
a) you're the only one who believes that is true.
BUT
the 'that' in each is grammatically different. in the first it is a conjunction; in the second it is a pronoun.
2) The only thing that passes, it is life, or
Rare, but used in poetry.
b) The only thing that passes, is life.
OK. but the comma is unnecessary.
Which of them comments. are
the correct
s.
Adjectives generally don't have a plural ending.
...