Why is は used in phrases with はず, if they are formal nouns that take a subordinate clause? Based on what I understand, formal nouns like take up a subordinate clause, therefore taking to mark the subject, but when I see examples such as > or > It makes it look like my understanding is wrong. When I tried to contemplate why, I just imagine that and aren't a part of the subordinate clause, but it is more of saying "that person/book is ". If that may be the case can this also be applied to other formal nouns such as ?

Technically, and are the short subordinate clauses that modify . Japanese relative clauses can be very short. As you said, the word marked with is working as a topic of the entire sentence.

lacks a topic, and this doesn't even look like a valid sentence to me. It's just a fragment roughly meaning "probable-ness of that person being young". Still, you can make the subordinate clause have a different subject using :

>
> He thinks that person should not be young.