I know it's to indicate whether something is a Na adjective or not, but other than the language rules stating it to be so, is there any real use of it?

I'm pretty sure everyone would understand you just fine with or without the extra「だ」so to me it's really unnecessary from a practical standpoint.

Is there a linguistics standpoint that makes 「だ」absolutely essential and not just a pointless language rule that everyone follow just because.

  • 3
    There are no such things as pointless language rules that people follow "just because." Also, just because people understand you doesn't mean what you've said is grammatically correct.
    – Kurausukun
    Commented Dec 9, 2017 at 9:15
  • A good way to approach this is by asking yourself "is the sentence complete without と思う appended at the end?" if it isn't complete, first complete it, then add と思う. That alone should point out why it's necessary.
    – psosuna
    Commented Dec 14, 2017 at 17:43

2 Answers 2


だ is a copula and it is a fundamental aspect of Japanese grammar and syntax - Copula

Leaving it out would simply result in an ungrammatical sentence like 「そうと思う。」It's a bit like asking "Do you need the 'is' in "he is a man" in an English sentence. You need it because in English the verb 'to be' functions as a copula. People would certainly understand the sentence without the 'is', right? But that's not how languages work. You need だ because the copula in Japanese indicates the relationship of the subject and predicate.


When と follows directly after a noun, it can be ambiguous if it stands for a case particle or a quote. In addition, although it can be omitted in casual conversation, it inevitably sounds a sentence with the verb omitted, hence not authentic.

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