What is the particle 'だ'? My student gave me a question shown in the title. And I showed here the answer. A nice question=) だ is, yes, the short form for 'です'. But it can be applied for the sentence of 'Xはnounです' or 'XはNA-adjectiveです' while you can not use だ in such way if the sentence is 'XはI-adjectiveです'. ex) わたしはカナダ人です and わたしはカナダ人だ. Both are fine because カナダ人 is the noun. 彼女はきれいです and 彼女はきれいだ. Both are fine because きれい（な） is な-adjective. But レッスンはたのしいです is fine while レッスンはたのしいだ is incorrect. To make it correct form, you shouldn't put だ right after い-adjective. レッスンはたのしい is enough in the short form. And だ-form is not impolite, rather, simply casual or shorter form. And it has a very important part in making the sentence more colloquial form. ex) あなたは日本人だね？(seeking confirmation) わたしはカナダ人だよ(wanting to make the listener assured) Does it make a sense?=)