Negative polite ending in compounding sentence


I am a beginner. In the TaeKim Guide textbook, there is a rule for the
compounding sentence:

> Positive: Conjugate the verb to its past tense and replace 「た」 with 「て」 or
> 「だ」 with 「で」. This is often called the te-form even though it could
> sometimes be 'de' .
>
> Negative: Same as i-adjectives, replace 「い」 with 「くて」.
>
> This rule also works for the polite 「です」 and 「ます」 endings.  
>  例） 学生です → 学生でした → 学生でして  
>  例） 買います → 買いました → 買いまして

What I want to ask is: What is the rule for the negative of 「です」 and 「ます」
endings?

From [Compound sentence and
politeness](https://japanese.stackexchange.com/questions/38292/compound-
sentence-and-politeness), an example is:

> 私は学生ですから勉強します。 ( I study because I'm a student )

How to change the sentence into "I do not study because I am not a student"?




Negative of です can always be ではありません. But here you have to be careful because
the notion of "negative" is quite different from what it is in latin-based
languages. Like you could ask "what's the negative form of X in
German/French/Italian" and you could get some usable information in real life
from the answer you might get, but that's not guaranteed in Japanese. So I
strongly caution against asking that question in the first place, if your goal
was to add to your conversational repertoire. That's a rabbit hole you don't
want to go down, beacuase people avoid the gramatically simple negative form
like plague and hence there are numerious ways around it instead. Imagine if
you will, a whole race of people avoiding "no"!

That said, you can say:

そうです。

The gramatically simple, correct negative to it is:

そうではありません。 そうではないです。

...but you could cause some drama you don't intend to, depending on the
context and who's around and what the subject it, saying that. Consider:

そうとは限りません。"that's not always the case".

As for negatives for ます、it's somewhat simpler. It's always ません。

As for turning 私は学生ですから勉強します。 into "I do not study because I am not a
student":

First, omit the subject because it's obvious. (don't state the subject like
_I_ unless you really, really have to. By deafult, don't.) Now you have:

学生ですから勉強します。

..and them, to answer your original question:

学生ではないので勉強しません。

would do.

