If there’s one thing in the English language that can be a cause of stress, it’s stress. Our words do not have a consistent pattern of stress, and sometimes it can’t even be readily guessed by looking at them: there isn’t anything within the form that shows where it goes, and so you need to have deeper knowledge of the word – or simply to have learned it by rote.
And sometimes you haven’t. Jim Taylor, in suggesting a look at this topic, mentioned a friend who pronounced cadaver as “CADaver”. Of course, only a cad would aver that a person who made such an error was somehow inferior; we can go much of our lives without hearing some words, and so it is not uncommon to be left to our own guesses. The result is that people, as is sometimes remarked, get the emPHAsis on the wrong sylLABle.
Ah, emphasis. It’s a way of saying which way the “oomph” faces. Actually, emphasis originally referred to putting more in a word than its denotation – that is to say, using it to imply something extra. From that it came to refer to intensity of expression, and so it is also sometimes used to mean what is also called stress or accent in words. Which is suitable, because if you have a foreign accent, you’re likely to find that the emphasis is, as mentioned above, a cause of stress.
But, now, why isn’t it “emPHAsis”? After all, it’s “emPHAtic”. Well, for this pair in particular, we need to go back to the source – Greek: emphasis has come to us unaltered from Greek, and in Greek, the stress – the emphasis, one may say, although in classic Greek it may have been more a question of vowel intonation than of emphasis – is on the first syllable. The word comes from em “in” and phainein “show” (we see the same root in, for instance, epiphany), and the rules of Greek morphology move the stress to the first syllable in this case. On the other hand, in the derived Greek word emphatikos, the added suffix puts the stress on the last syllable – a syllable which we have dropped in English emphatic, and we’ve put the stress, as we generally do in -ic words, on the second-last syllable (the penult, as it’s called).
So what this word has within it that guides its stress is its Greek origin. And origins turn out to have a lot to do with English word stress. English has gotten its wordstock from a lot of different languages, and different languages have different rules about stress – and we sometimes, though not always, keep the stress when we borrow the word.
Some languages, like Greek, have stress that shifts and that is not really consistent from word to word – on the other hand, Greek also has accent markers to show which syllable gets the stress (though in modern Greek they’re not always used). Some languages have stress that is contingent strongly on vowel length – Latin has this characteristic. Some languages have stress and vowel length independent of each other: Finnish and Hungarian both always put the stress on the first syllable regardless of which vowels are long or short. Some languages have consistent stress without contrastive vowel length: Polish, for instance, always has stress on the penult, but doesn’t really have a long-short vowel contrast.
English, for its part, does have some predictable patterns, and to some extent they relate to what we call “long” and “short” vowels – though in Modern English the distinction is one of quality rather than of quantity (long vowels in English actually were, more than half a millennium ago, just extended versions of the short ones, but that all changed during what’s called the Great Vowel Shift). But much of that actually relates to the other languages (especially Latin) that we got the words from. In truth, Old English (which is what was spoken in England from roughly the 7th to the 11th century AD) put the stress as a rule on the first syllable of the root. (If there was a prefix tacked on the beginning, it wouldn’t receive stress.)
What this also means is that when we add a suffix that came down from Old English, it will probably not affect the stress of the word. Suffixes such as -dom, -ful, -hood, -man, -ness, -ward, and -wise don’t draw stress to them – though -wise with its “long” vowel gets a secondary stress. On the other hand, suffixes from French, such as -ee, -eer, -aire, -elle, -esque, -ese, and -ette, tend to grab the stress.
Meanwhile, words we get from Latin – such as cadaver – tend to get stress on syllables that in Latin had a long vowel (in cadaver, the second /a/ was long: /ka da: ver/). Words that come from Greek sometimes follow Greek stress patterns, and sometimes just get the accent on the antepenult (the third-last syllable) in that grand old English habit: Socrates was said in Greek with the stress on the penult, but in English we shifted it to the antepenult. Words we got from French, if we present them as French words not much changed (more typical with more recent borrowings), will get stress on the final syllable, but there are many words in English that came from French long enough ago that the stress has changed. And of course those words usually are derived from Latin words, which will add further influence.
So how do you know what syllable an English word is stressed on – which syllable gets the emphasis, as it were? Dude, look it up.
But, ah, if you’re on a desert island with no internet and there’s a maniac who’s going to hurt you if you don’t know the pronunciation of this or that word (in real life there are many more such maniacs than there are desert islands to put them on), start by identifying the bits of the word. You can ignore all the inflectional suffixes – things like -ing and -ed and -es and so on – but of course you will need to take note of any suffixes that tend to draw the stress (including -ic, -icity, and -idity, which draw it to the syllable before them).
If the root has two syllables, the stress is probably on the first – unless it’s a loan word or one of those verbs that pair with adjectives and nouns (insult, perfect, torment, escort, etc.).
If the root has more than two syllables, look at the penult of the root. If it has a “long” vowel or the vowel is followed by two or more consonant sounds before the next vowel, it’s probably stressed. If not, the stress probably goes on the antepenult.
Unless it doesn’t, of course.