Tobberoth Wrote:. . . the second one isn't actually のだ as explanatory, it's ん turning 飲む into a noun and じゃない saying "it shouldn't be so".
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That was a very succinct and helpful explanation. Thanks.

The 初級 edition of 日本語文法ハンドブック has a lot of pages about のだ and not all of it is covered in that volume... There was more to it than I initially thought.