latinplural-forms
What is the plural of octopus? Is it octopuses, octopi or octopodes?
Normally in English it's supposed to be octopi, but when I type it, it has a red line under it (by the spell check), which means that it's incorrect. I also saw the word octopuses in my school's textbook. I got really confused about which one is correct.
Thanks! Any help would be appreciated.
Both are correct, though there are some specific usages as pointed in another answer; Google Ngrams suggests that indices is slightly more used than indexes.
For American English we have (in both charts, indices is blue and indexes is red):
For British English we have:
Although the proper Latin plural would be fora, forum has been adopted into the English language--and in most cases follows the rules of English pluralization. Similar changes can be seen with the adoption of other words like octupus (the proper plural would be more like octopedes, but in English we usually say either octopi or octopuses).
The current entry from the Oxford Dictionary says:
The plural of forum is usually spelled forums; the plural fora (as in the original Latin) is chiefly used when talking about a public square in an ancient Roman city.
Since you're talking about online discussion boards, I would use forums instead of fora when you are talking about more than one. An English speaker might understand fora, but it wouldn't come as naturally.
Best Answer
From The Corpus of Contemporary American English (COCA) and The British National Corpus (BNC):
The most common plural is octopuses. Use that.
The alternative octopi is also somewhat common. In fact, it's common enough that we should probably consider it standard as well, but because octopus isn't actually derived from Latin, pluralizing it this way may rankle some feathers. If you'd like to avoid criticism, avoid using this form.
We'll discount octopoda entirely since all four results are from Orson Scott Card's Lost Boys, in which fictional characters discuss the proper plural of octopus.
The least common on the list is octopodes. Although you can make an argument that this plural is more correct than octopi, the fact is few people know it and fewer people use it. If you pluralize octopus this way, you'll draw more attention to how you're saying it than what you're saying. Octopodes probably isn't common enough to be considered standard, and worse, it may fail to communicate entirely.
The verdict? If you want to communicate, use octopuses. Save the other plurals for when you want to use words like toys.