For this sentence 本日は晴天なりといったとする, could I replace なり to です, if not, why?


ChatGPT said '"なり" (nari) functions as a copula, which is a word that links
the subject of a sentence with a predicate or description. Specifically, "なり"
serves as a literary or formal version of the copula "だ" (da) or "です" (desu)
in Japanese.' Is this correct or not in this sentence?




It's very misleading to say なり is a _literary_ or _formal_ version of だ. It is
an _obsolete_ copula that is no longer actively used in Japanese. You'll never
come across it in contemporary Japanese, but you may find it if you try to
read old documents on display in a museum written by samurai.

However, 本日は晴天なり is one of the rare exceptions where you might encounter なり in
modern Japan. This phrase is widely known as a set phrase used to check
microphones or radio transmissions. An English equivalent would be "Mic check
one two", "Radio check" or [Harvard
sentences](https://en.wikipedia.org/wiki/Harvard_sentences). Since 本日は晴天なり is
more of a "test code" rather than an actual message, changing it, such as
replacing なり with です, would defeat the main purpose of the phrase. You must
always use this phrase verbatim, even if it's heavily snowing outside.

(By the way, while you don't need なり to read modern texts, you do need である,
which is a formal and literary version of だ.)

