....だと思う and ...と思う


When is ...だと思う and when ...と思う used? Do I understand correctly that the
former is used after a noun and na-adjactive only, and the later after a verb
and i-adjective?




I like this question a lot, and I think we can answer it with a better
understanding of how the と particle works.

In this case the「と」particle is actually a quotation particle. The formation
for use of this particle is relatively easy:

> (quoted sentence with plain form ending) と (Your verb. i.e. 言う、思う、聞く、etc)

So really what comes before the end of the と particle in this case is the
plain form verb. I think that several examples will help you out.

> 彼女｛かのじょ｝は綺麗｛きれい｝です。(She is pretty)

綺麗｛きれい｝is a な adjective. Using it at the end of a sentence, we get です, or in
plain form だ instead of な。

> 彼女は綺麗だと思います。(I think that she is pretty.)

Taking the first example sentence, and putting it into plain form, we can add
と思う。 Without any complications.

> あのビルは大きいです。(That building is big.)

大きい is, of course, an い adjective. In the sentence I ended it with です (to be
polite), but です in this case is unnecessary. I could say `あのビルは大きい` and still
have it mean the same thing. Plain form with い adjectives actually end with
the adjective, which in this case is 大きい。

> あのビルは大きいと思います。 (I think that building is big.)

I'll give you a few more examples without commentary, to hopefully solidify
the point.

> あの人は大学生です。(That person is a college student)
>
> あの人は大学生だと思います。(I think that person is a college student.)
>
> 彼は十キロ走っています。(He is running 10 kilometers.)
>
> 彼は十キロ走ってると思います。(I think that he is running 10 kilometers.)

