Your choice of example is slightly unfortunate since there is an element of expectation involved. Simply using not is correct since
She is supposed to be happy.
describes an expectation you have, in reality she may or may not actually be happy, so the opposite is
She is not supposed to be happy.
which is the negative of your expectation. The expectation is created by using supposed.
It is supposed to be hard. (expectation)
It is not supposed to be hard. (negative expectation)
Tomorrow is supposed to be day off. (expectation)
Tomorrow is not supposed to be day off. (negative expectation)
Tomorrow is not a day off. (certainty)
Saying
You are not supposed to smoke here.
is a negative expectation of something you are not allowed to do, but you would only be told this if you were smoking or about to smoke.
To be forbidden to do something, you would be told
You are not allowed to smoke here.
just as
She is not allowed to be happy.
forbids her from being happy.
Hopefully this helps.
The subject in this sentence is the person referred to by the pronoun "she" -- in this case, this is Beverly. I'm not 100% clear on your thought process, so it's difficult to provide much guidance on how to identify this beyond the fact that "she" is a nominative pronoun and is in the typical position for the subject of a sentence.
As for "it", this refers to what she's just said, and harkens back to her earlier line:
There’s something I should probably say
What she just said, that Dana and Matthew were arguing, is the "something she should probably say".
Best Answer
"Today only" means "Today is the only day that you can submit it." You will often find it in advertising: "Today only! All fish at half price!"