When a language borrows a foreign word the word goes through a period of naturalisation. This can take a long or short time depending on how different the two languages are and how tolerant the new language is of non-standard words.
The word starts off as a conscious foreign borrowing, then later becomes a normal, but noticeably foreign word, before finally becoming accepted as a local term.
In Japanese, for example, an English word is immediately respelled to match Japanese pronunciation rules. French leaves English words alone for longer before finally (and inconsistently) making changes to them.
English, however, rarely changes foreign borrowings to force them to match English spelling an pronunciation rules, and only very slowly forces them to comply with our grammar.
"Czech", for example, is the only word in English spelled with a cz and there is no sign of this changing. French spells this word "Tchec".
Note that naturalisation is not dumbing-down; it is a perfectly normal development observable in all languages.
In the case of Latin words like forum, it has a long history in English. It is at this stage an English word, not Latin. Fora is its original plural in its source language, but this is only really of interest to linguists. Forums is the correct plural to use. (Note that "Octopi" is a spurious plural for the word octopus; this word is Greek, not Latin, so its correct original plural is octopodes, and its correct plural in English is octopuses.)
Note that the plural form which is used more often might not necessarily be the correct one.
English is very laissez-faire in this respect, too much so in my opinion. We don't enforce our own rules often enough and even when we do it is in a lackadaisical manner. For example, indexes and indices are both used often - we should just be standardising on one.
The poster that noted the unpleasant plurals radiuses, indexes and matrixes is due to a particular problem with euphony here - these words all end in s (x is just "ks" written with a special letter) and adding "-es" to such words results in clunky pronunciations.
Originally Posted by Janey_Mac
Disclaimer: I love indices, matrices, fora, referenda and (god help me) octopodes.
I love words that are out of the ordinary, I love working out the rules that they're abiding by.
That's fine - so they are of interest to you because you're interested in linguistics (I assume). Does that mean they should be officially spelled that way too? Just so linguists can have something to discuss? Surely not! Languages are simply a vehicle for expressing thoughts. Their grammar and spelling is a functional, not artistic, part of that system.
Ideally the only acceptable plural of forum would be forums and a dictionary would state "Original Latin plural 'fora' ". This way interested parties could find out the word's history but regular people wouldn't need to know. English is fast becoming the world's lingua franca and it is out of our hands now. We need the lingua franca to be better structured.