politeness with と思う


I've been reading Minna-no-nihongo and I came across examples:

> 便利 **です** が物価が高いと思います。

> よく勉強し **ます** が、本を読まないと思います。

  1. For some reasons I thought that an idea or opinion expressed with と思う had to be in informal form. But in those particular examples there are です and ます. Even if I was wrong and you can use polite form there then why one clause is in polite form and other is in inpolite one? Shouldn't two coordinated clauses be in agreement regarding politeness? So, question is:

What is the rule for politeness level with と思う? Something like 'you should put
in polite form only the last clause preceding the と思う' or something like that.

  2. I was thinking that one option to explain it grammatically is that と思う in those examples refering only to second clause. So, it's not

> I think that they study well but don't read much.

but more like:

> They study well, but I think they don't read much.

If that's the case then is it possible to use:

> よく勉強するが、本を読まないと思います。

with the meaning: "I think that they study well but don't read much", where
all the expression preceding と思う is my opinion?




Basically, yes - you only need to put the last clause, the bit ending right
before the と of と思う, in plain form. It doesn't strictly have to be in plain
form, either, but it sounds much more natural to do so, unless you're trying
to sound extra-polite.

But I also think that your argument that only a portion of the sentence is
being "quoted" by the と is also a valid way of thinking about it, at least in
some cases. Certainly, I feel that "They study well, but I think they don't
read the book" is a more natural translation of the above than "I think they
study well but don't read the book". However, I don't think reducing the します
to する would be the appropriate way to indicate that - it wouldn't convey that
inclusion any more clearly, but would just reduce the overall level of
politeness/formality.

Instead, I think it'd be more natural to just state the thought completely,
and then follow it with "Or anyway that's what I think."
彼らはよく勉強しますけど、本を読みません。まあ、そうと思いますけどね。(But if I'm being honest, I don't think
either this version nor the original sound very close to something an actual
native Japanese person would express - it's more "textbook Japanese".)

Note that while people often speak of 読みます being "the -ます form of 読む", the
reality is that ます was originally just an auxiliary/helper verb with no
particular meaning (that I'm aware of), added for the purpose of expressing
politeness or deference. In regular Japanese 丁寧語, it mostly suffices to pepper
in at the ends of major clauses. You don't use it in modifying clauses, and
really the reason why you don't use it before the と of と思います is really just
because you're already using -ます for the 思う, so you don't need to use it twice
so close together. At least, that's how I think about it, dunno how valid it
is.

