English Grammar questions? Please?

Okay, English learner - you have several questions to ask - you're asking for answers.
1) No - it's not that simple. Adjectives and nouns are very different. A few can be used as both, but not many.
2) Yes - people use both versions.
3) No - that depends on the word and what in what sense it's being used.

I’m an English learner here in Taiwan.
In your question. 1).
I just know “elderly” and “disabled” both are adjective, but by using articles in front of it, it is assumed as a noun.
In your question. 2).
I’m little confused by the answer. Yes I can Google to know “people use both versions.” But there are lots of people insist only one way is correct, which you said in your question.
Evidences exist in this website, which a question posted 5 years ago can easily be seen as follows:
“They must have not known or they must not have known?
Which is the correct sentence? 1 or 2?
(1). They must have not known what it meant.
(2). They must not have known what it meant.
Best Answer: the second one
Jessica · 5 years ago”.
I just copy some sentences from these website as follows:
“your initial question asked about "must have not
to", but the example text reads "must not have to". I don't
think I've ever really seen "must have not to" used in any
English text so I would assume that is a typo”.
In your questions 3. I know there are quite a few words, just be added with “less”, then the word become an adjective, examples like：
care --> careless,
soul --> soulless,
time --> timeless,
end --> endless,
relent --> relentless,
change --> changeless,
There must be many more I can’t list it.