I keep noticing the spelling mistake habeus corpus (instead of habeas corpus). The explanation is straightforward: the final syllable of habeas is usually pronounced in English just like the final syllable of corpus, namely as əs. If it’s pronounced the same, there is an obvious temptation to spell it in the same way.
The word habeas is the second person singular present subjunctive active (‘thou mayst have’) of habeō, habēre, to have. Its final vowel was long, habeās, so when I was taught Latin at school we pronounced it ˈhæbeɪɑːs (or ˈhæbiɑːs). English legal Latin gets both the initial and final quantities wrong, producing ˈheɪbiəs. But then legal Latin is something of a law unto itself, or lēx in sē as we might say.
The reason our Latin and Greek teachers were so insistent on our “getting the quantity right”, i.e. distinguishing between long and short vowels, is that classical versification is based on the idea of heavy (‘long’) vs light (‘short’) syllables. This is important in the appreciation of Latin poetry (see my blog for 18 Aug 2006), but was even more important for us schoolchildren who were expected to compose Latin verse every week, usually in the form of translating a passage of English poetry.
We had a useful book called a gradus, short for Gradus ad Parnassum, ‘a step towards Parnassus (the mountain sacred to the muses)’— Ainger A.C. and Wintle H.G., An English-Latin Gradus, London: John Murray, 1890. The gradus was a dictionary with a difference. It specialized in providing sets of homonyms or near-homonyms, all with the vowel quantities clearly marked, so that we could select a particular translation that would fit the scansion we wanted. Poetry translation became a kind of jigsaw puzzle, finding words that would fit the metre. A hexameter line consisted of six feet, each of which was either a dactyl (ˉ ˘ ˘) or a spondee (ˉ ˉ). So if you wanted to put ‘money’ at the beginning of a line of verse, you could choose argentum, aurum, nummus, res, or divitiae; but not the commonest equivalent, pecunia, and not opes, bona or moneta.
I don’t suppose many schoolchildren have to do that sort of thing nowadays.