Does using 「だ」in だと思う actually serve a practical purpose?


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.




だ is a copula and it is a fundamental aspect of Japanese grammar and syntax -
[Copula](http://www.japaneseprofessor.com/lessons/beginning/the-copula-desu/)

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.

