> "John is" is not contracted to "John's"

John's not sure he agrees.

> "John is" is not contracted to "John's"

It is, actually. It's just best avoided due to confusion unless you're doing it for quoting or dialogue purposes.

https://www.writersdigest.com/write-better-fiction/contracti...

Fun historical fact: "John's knife" was once a contraction of "John his knife".

No longer of course, the latter isn't grammatically correct in modern English. But that's where it comes from.

This is not true, it came from an inflectional ending in Old English. https://www.merriam-webster.com/words-at-play/history-and-us... uses the example “cyning” (king) and “cyninges” (king’s). In theory the apostrophe stands for the missing e.

Goes to show I shouldn't trust something, just because it was taught to me in school...