I’m sure most of you have learned the expression 「そう」 by now. Now, there are four ways to make a complete sentence using the state-of-being with 「そう」 to produce a sentence that says, “That is so.”
Different ways to say, “That is so.”
- そう。
- そう だ。
- そう です。
- そう でございます。
The first 「 そう」is the implied state-of-being and 「そうだ」 is the declarative. As I’ve stated before, the non-assuming soft spoken 「そう」 is often used by females while the more confident 「そうだ」 is often used by males.
「そうです」 is the polite version of 「そう」. 「そうです」 is not the polite version of 「そうだ」.
Different ways to ask, “Is that so?”
- そう？
- そうか？
- そうですか？
「だ」 is used to declare what one believes to be a fact. Therefore, 「そうだか？」 is not a valid way to ask a question because it is declaring a fact and asking a question at the same time. But the fact that 「そうですか」 is a valid question shows that 「です」 and 「だ」 are essentially different.
Besides the difference in nuance between 「だ」 and 「です」, another key difference is that 「だ」 is used in many different types of grammar to delineate a relative clause. 「です」, on the other hand, is only used at the end of a sentence to designate a polite state-of-being.
- そうだ と思います
I think that is so. - そうですと思います
(Incorrect sentence)
「そうだと思います」 is valid while 「そうですと思います」 is not because 「です」 can only go at the end of the sentence.
For previously explained reasons, you must not use the declarative 「だ」 with the question marker.
キムさん：イタリア料理を食べに行きませんか。
Kim-san: Go to eat Italian food?
鈴木さん：すみません。ちょっと、お腹がいっぱいです。
Suzuki-san: Sorry. (My) stomach is a little full.
Japanese X Benkyou 