When does omission of です constitute casual speech?


In many textbooks, polite style speech is taught, which means です and ます
endings are introduced, and used in many sentences.

However, I have encountered sentences with omissions, for example sentences
which omit verbs. They may also end with particles. I think this is done to
introduce vagueness, or simply to make the sentences shorter.

In what context do such omissions constitute not polite speech anymore, but
**plain speech**? For example, in the drama [Meguri Ai, Episode
8](https://youtu.be/mNOOEfpjuUM), there are some "incomplete sentences" that
have differing levels of politeness _(I have some context and sentences
written here, but if more background is desired, the specific video segments
are listed below - please visit<https://youtu.be/mNOOEfpjuUM>)_:

Video time: 02:35 - 03:30:

> 社員： **おつり、いいから。** (plain)
>
> A boss (社長, name 修二) is in the back of a taxi with his subordinate (社員), and
> the subordinate says this to the driver. Although polite forms were used
> with the driver up until that sentence, I feel that this sentence is in
> plain form as the subordinate feels superior to the driver due to a
> perceived difference in social status.

Video time: 11:25 - 11:45:

> 修二： **じゃあ、早稲田まで。** (polite)  
>  絵里（運転手）：この時間だと電車で行くほうが早いとおもいますけど。。。(polite)  
>  修二：いいんです。行ってください。 (polite)
>
> Apparently this sentence is still polite, according to my teacher. I am not
> sure why, though.

Video time: 12:53 - 13:10:

> 修二：そういうわけにはいかないんです。 (polite)  
>  絵里（運転手）： **どうして** ？ (plain)  
>  修二：そいつにだけは、幸せになってほしい **から** 。そうおもって２年前に別れた **から** 。(plain)

Eri (絵里) asks どうして, which apparently is in plain style, and Shuuji (修二)
replies in plain form with two occurrences of から without です.

There are many sources that state that [ending with から is
informal](http://yesjapan.com/YJ6/question/1190/would-you-say-kara-desu-or-
desu-kara). However, まで is another particle, yet ending with まで without です
(second bolded sentence) seems to leave the polite style speech unaffected.
Again, どうして without です seems to be informal.

* * *

**When does omission make sentences casual / plain?** Examples would be
greatly appreciated.




I think じゃあ、早稲田まで isn't polite but plain. じゃあ、早稲田まで行ってください and じゃあ、早稲田までお願いします
would be polite.

Sentences without です, ます aren't polite, so the sentences that end with
particles like 早稲田まで, ほしいから, and 別れたから aren't polite. And どうして？ isn't polite
either because it's without です, ます.

I think omission of です, ます always makes sentences casual / plain.




I think じゃあ、早稲田まで isn't polite but plain. じゃあ、早稲田まで行ってください and じゃあ、早稲田までお願いします
would be polite.

Sentences without です, ます aren't polite, so the sentences that end with
particles like 早稲田まで, ほしいから, and 別れたから aren't polite. And どうして？ isn't polite
either because it's without です, ます.

I think omission of です, ます always makes sentences casual / plain.

