How to use ～に同意する to agree or disagree with a person?

Question

A couple of times I've tried to say "I agree with..." or "he agrees with..." but I don't know whether I'm using 同意{どうい}する correctly.

In what ways should に同意{どうい}している be used? Also, which particles should be used? Can only に be used before 同意{どうい}する or are there circumstances where other particles are used as well, such as と and を?

In many examples I've found using 同意{どうい}する, I've seen （だ）ということに or just （な）ことに (the だ/な after na adjectives/nouns and without だ/な after i adjectives/verbs it seems.) Is there a specific reason for using one over the other, e.g. is the longer （だ）ということに more polite, and what do these mean?

Some example sentences:

• I agree with his opinion:

  彼{かれ}の意見{いけん}に同意{どうい}します。

• I agree with him that oranges are tasty:

  オレンジはおいしいということ彼{かれ}に同意{どうい}します。

• He agrees that apples are tasty:

  彼{かれ}はりんごがおいしいことに同意{どうい}します。

• Many people agree that brussel sprouts don't taste nice:

  芽{め}キャベツはおいしくないということに同意{どうい}する人{ひと}が多{おお}い。

See also: What are the different ways of saying to agree or disagree with a person? for other ways of agreeing/disagreeing with someone.

Answer 1

You can find a lot of examples here (you may want to check later pages), which will explain a lot.

It seems that we do use "に" for "同意する".

"ということ" literally means "the so-called", which I think is a little more polite.

Also to @sawa, search in the above site with "いという" and "いだという" (I know they are not words nor phrases ...) gives interesting results. It seems that wile i adjectives, "美味しいだということ" can be used.

And I think "オレンジはおいしいということ彼に同意します" is still a little weird, maybe it should be "オレンジがおいしいだということについて彼に同意します。". A phrase with nothing in the end doesn't sound natural. (About "は" and "が", I just feel this way, I am not able to explain it in details.)

BTW, I am not native, so correct me if I am wrong.