From someone who uses Latin every day and got corrected by someone (who had NO idea who he was correcting) because I said Focuses:
Focus, Foci would be a second declension (masculine) noun in Latin.
Nominative - Focus. The subject noun of a sentence is nominative. Example: Focus currus Ford est" - "Focus is a Ford car". Car is being used as an adjective here, not a noun, so it uses the same case as what it is describing, the Focus.
Genitive - Foci. Genitive is possessive or indicates origin or a quality: Example: "Rota Foci est" - "The wheel is the Focus's"
Dative - Foco. Dative is the indirect object. Example: "Cletus dedit rotam unam Foco" - "Cletus gave a wheel for the Focus". Notice how not only did the word for Focus change, but the first declension noun "rota" (used as a direct object) changed to rotam (accusative case - as did the adjective for "a" or "one" wheel, una change to unam, matching what it was describing, the direct object).
Accusative - Focum. The accusative case is for the direct object of a sentence. Example: "Cletus posuit Focum in stabulo" - "Cletus put the Focus in the garage".
Ablative - Foco. The ablative is most often used for prepositional objects, like stabulo ("stable", substituting for modern word "garage") in the example above.
Example: "Cletus ferri rotam in Focus" - "Cletus carried a wheel in the Focus".
OK, now we do PLURAL, more than one. We start all over!
Nominative - Foci. Example: "Foci curri Ford sunt" - Focuses are Ford cars". Notice two things: The verb "est" (he/she/it is) becomes "sunt" (they are) and we aren't declining the noun/adjective Ford anywhere, since it is a modern English word. That's just the way some latinists do it. There might be purists who would translate "Ford" into "Vadum" (2nd decl. neuter) which is the Latin for the place one would ford a river (and a likely etymology for Henry Ford's last name). I don't bother with those things.
Genitive - Focorum. Example: "Rotae Focorum sunt" - "They are Focuses' wheels". Literally, "Wheels are from (of) the Focuses".
Dative - Focis. Example: "Cletus dedit rotis Focis" - "Cletus gave wheels for the Focuses".
Accusative - Focos. Example: "Cletus posuit Focos in Stabulo". You can see that Cletus's garage is getting full.
Ablative - Focis. Example: "Cletus ferri rotas in Focis". Cletus carries wheels in Focuses".
So, if I was to say, (as I may have in the thread that provoked this) "I am going to wash both of my focuses" and one would correct me to say "I am going to wash both of my Foci" well, they're being an arrogant ignoramus, because they're presuming to correct someone's use of a Latin-derived word when they don't have the foggiest clue of Latin grammar. And yes, that kind of thing sets my teeth on edge and gets me to spend half an hour creating a thread like this. If they had corected my sentence to "I am going to wash both of my Focos" I would have said "well done" and moved along. As it turns out, I've ended up flaming this guy several times in that thread because, as I have said, he's an arrogant ignoramus and it carries over into the rest of his ideas. Idiots are usually consistent.