One hippopotami cannot get on a bus, 
Because one hippopotami is two hippopotamus.
If “TARDIS” were a Latin word, it’d presumably be 3rd declension, and would probably have the plural “TARDES”. “TARDII” would be the plural of “TARDIUS”.
I would not consider “f turning into v and then add -s or -es”, “singular same as plural”, nor “loanword that keeps the pluralization rules of its original language” to be irregular.
“Children”, “feet”, and “octopi”, however, certainly are irregular.
There are only three words in English where the last three letters are arf - dwarf, scarf, and wharf. In each of them, sometimes the plural ends in arfs and sometimes ends in arves. At sone point centuries ago, the plural always ended in arves for those three words. The definition of an irregular noun is one where the plural is not made by just adding s or es to the singular. That is the definition of it:
https://www.microsoft.com/en-us/microsoft-365-life-hacks/writing/what-are-irregular-plural-nouns
The rule isn’t “replace arf with arves”. The a and the r are irrelevant. The rules is to replace the f with a v, and then add an e if there isn’t already one, and then an s. There are other words that follow that rule, such as “staves” and “knives”.
An irregular plural is, by definition, one that doesn’t follow the rules. English has multiple different rules that can be followed. Adding an s or an es is the most common rule, but same-as-singular and same-as-in-original-language are also valid English rules.
And these are also irregular nouns, since they do not just add s or es. Note that the website I linked to only said that it was the 100 most common irregular nouns, not all of them. That’s the definition of irregular nouns. You can make you own definition if you want, but then you’re not using the standard definition. Staves is, incidentally, the plural of staff. Staves may have once been the more common plural, but now it’s staffs. Knife usually has the plural of knives (although some people use knifes). Again, I’m using the standard definition of irregular nouns.
No, it’s a word that doesn’t add s or es to make its plural.
That can’t be the standard definition for an irregular plural, because by that definition, almost all words in almost all other languages would be irregular. The standard definition for an irregular plural is a plural that doesn’t follow its language’s rules.
In those other languages, they make plural words by their own definition. The rule that adding s or es makes a plural only applies to English. It’s not possible to apply the rules of a different language when you’re talking about what’s an irregular word for any part of speech.
But the -s- in island is under the influence of Norman French, not Latin. French does have a number of silent letters added etymologically from Latin.
Also, sorry, I’m not watching a video. I’ll happily read a proper source, but I think I know more about this than anyone making a popular video.
Sorta-
In Middle English, it was spelled iland or yland. Later, English scholars mistakenly assumed that the word came not from its Germanic source, but from French isle, or “island.” They began spelling the English word as isle-land, and by the 17th century island. The French word was previously spelled ile, but scholars suspected it derived from the Latin word for “island,” insula, and added the letter S to make it look more like Latin.
So, yes, the silent S was added to male it look more latin, and thus “better”.
Rob Watts is an etymologist and journalist, who worked for BBC. Yes, I agree with YTs not being the best sources, but some are quite educational.
This was actually the answer for a Jeopardy clue tonight!
In Latin times this noun referred to the punishing of every tenth man; now it refers to any wholesale slaughter
Barf.
Technically, “arf” is onomatopoeia for the sound a dog makes. All those bow-wows and arf-arves, it was cacophony!
Snarf.
I do think there are a few examples where the silent letter were added into English after a standard pronunciation and spelling had solidified solely because the root word in Latin had that letter.
I think both receipt and debt are examples of this. Receipt never had a “p” in it, and it was never pronounced with a “p” all the way back to Middle English. But scholars realized that it came from recepta in Latin in the 1500s and added a "p’ to it, even though it was never pronounced.
Similarly for Debt from debitum. Always pronounced “det”, and spelled that way too until the 1500s.
Yes, but (A) many of these silent letters were added in French, not in English, and (B) consider “the ghost writes through the night; two knees”—lots of silent letters, but none there from Latin.
Some silent letters come from Latin, either directly or through French, but that’s not why English has all its silent letters, which was the original claim.
Oh?
What are your qualifications?
O.K., but note that that isn’t a noun. It’s a verb. It doesn’t have a plural.
“Barf” is also a noun. There’s barf all over the bathroom floor. Plenty of nouns don’t have a plural form (uncountable or mass nouns). Surely you knew that.
“Snarf” is a verb only, not that that was one of your stipulations.