Why is "f" in a singular replaced with "v" in its plural? [duplicate]

Solution 1:

This is a throwback to Old English. The "f" would be pronounced like a "v" if it was between two vowels.

For example, the word for heaven was heofon and would have been pronounced something like "hayovon" (Sorry, no IPA).

Here's some information from an article on English plurals:

In Old and Middle English voiceless fricatives /f/, /θ/ mutated to voiced fricatives before a voiced ending.[6] In some words this voicing survives in the modern English plural. In the case of /f/ changing to /v/, the mutation is indicated in the orthography as well; also, a silent e is added in this case if the singular does not already end with -e:

(Adding screen snap here because the markdown here doesn't do tables easily.)