We know you know how to make something plural: usually, you add an ‘s’. (Or in some cases, like X, as ‘es’.)
Is data already plural? Do I have one index, but many indices? What do you do with words like ‘vertebra’ or ‘attorney general’, if there’s more than one?
These probably won’t come up much. But if they do, here are a few tips:
If it’s a word that we use a lot, like ‘data’, think of it like an English word (because it’s become one!). Only the very old-school among us say ‘the data are showing…’ and refer to a single point as a ‘datum’.
If it’s in a product or service name (for instance, the indices of a S&P Global), follow their lead.
If it’s a word we use less frequently, like cactus or vertebra, you can go with Latin plural or the English one. If you go with the Latin, -us endings generally become -i, -um become -a, and -a become -ae. (Don’t bother with the exceptions. There are very few.)
Phrases like attorney general and court martial turn into attorneys general and courts martial. It’s called a ‘French plural’. Chances are, you’ll never have to talk about these, much less make them plural. But if you do, the person you’re writing to will probably care (they tend to be fussier, old-fashioned words).
Octopus has three accepted plurals: octopuses (English), octopi (Latin), octopodes (the original Greek).