This is the first in a series of short articles on English pronunciation and the teaching of English pronunciation to non-native speakers. This first article in the series concerns what I view as one of the most important aspects of English pronunciation, one that receives the least structured attention: syllable stress in multi-syllabic English words.
English is a stressed-timed language, as opposed to being a syllable-timed language (such as French, Spanish—every syllable takes up the same amount of time) or a mora-timed language (such as Japanese, where each vowel of consonant-vowel pairing or a vowel-vowel cluster takes up the same amount of time). In English and other stress-timed languages, stressed syllables occur at regular intervals throughout regular speech. Unstressed syllables occur in between. Stressed and unstressed syllables take up different lengths of time (stressed syllables typical being longer and more audible).
Phonologists classify syllable stress or lack of syllable stress using varying tiered systems of classification. One is a four-way stress classification: primary (tonic stress), secondary (lexical stress), tertiary (unstressed full vowel) and quarternary (reduced vowel); another, and perhaps one which is more useful for the purpose of language teaching to most English language learners, has two levels, stressed syllables and unstressed, whether the unstressed syllable is reduced or not.
It is important to take a moment to look at reduced vowels; they may be viewed in an informal way as the antithesis of the stressed syllable, that is, lying at the other end of emphasis they receive in the pronunciation of a word. Reduced vowels in a multi-syllabic word occur in syllables which are unstressed. This absence of stress in English causes the vowel to “reduce” to a schwa sound [ə]. Reduced vowels (schwa):

Examples with different words

a in ago or canary

e in define or terrain

o in demon or offend

u in suspend or until

i in decimal or implicate

y in syringe or vinyl
There are “rules” in English for syllable stress. However, these aren't actually rules, as they are descriptive rather than prescriptive. To prove that point, ask any native speaker to pronounce the word “imitation” by putting the primary stress on a syllable other than the third. Your native speaker will not find it easy, as the syllable stress is such an integral part of the word's pronunciation that it cannot easily be varied without making the word incomprehensible. Native speakers were not taught syllable stress; rather, they learn it as part of the normal pronunciation of a word, by listening to others pronounce the word.
Word stress rules (this list is not exhaustive nor is any attempt being made to present it as exhaustive; however, it provides a good framework for understanding that there are some general patterns to syllable stress in multi-syllabic English words.)

1. Stress on 1st syllable:

Most two-syllable nouns, such as label, format, interest, pity, treaty, purchase

Most two-syllable adjectives, such as lucky, grateful, handsome, fearful, active, skittish

Compound nouns, such as blackboard, sidewalk, streetlight, shoelace, backhand, headset

2. Stress on 2nd syllable or last syllable of a two-syllable word:

Most two-syllable verbs, such as invent, reply, decide, persuade, divulge, conduct, implore

Compound adjectives, such as run-down, close-cut, high-strung, pumped-up, dim-witted, ill-fitting

Compound and two-word (phrasal) verbs, such as withdraw, undo, pass out, give up

3. Stress on penultimate syllable (2nd to the last)

With words (adjectives) ending in –ic, such as syllabic, epidemic, intrinsic, autocratic, historic

With words (nouns) ending in –sion or –tion, such as distribution, decision, intuition, prevention

4. Stress on ante-penultimate syllable (3rd to the last)

With words (nouns) ending –cy, -ty, -phy, -gy, such as democracy, entity, photography, energy

With words (adjectives) ending in –cal, such as medical, surgical, practical

5. Stress on first syllable in compound nouns

hair brush, hay fever, pot holder, wind tunnel
In many case, word stress simply must be learned, without rules to do the teaching. It is helpful to teach students that the stress in multi-syllabic words will cause the stressed syllables to be spoken more audibly and with a bit more length than other syllables. The sounds on unstressed syllables will be muted or unclear. The fact that imperfect pronunciation of these muted or unclear syllables doesn't greatly impede a word from being understood tells us much about the importance of stressed syllables in the understandability by native speakers of many English words.
I personally don't think it is necessary to do any more than introduce these “rules” to students. I don't recommend that you rely on rules to do your teaching, even for grammar. (You may want to have your students discover some of these rules on their own.) Native speakers do not learn these syllable stress rules; and native speakers also have some difficulty placing the stress in unfamiliar multi-syllabic words (such as hegemony, not a commonly used word among native speakers, and which according to my dictionary can take the primary stress on the first or second syllable). A greater problem still for students, and for young native speakers (teenagers for example) is handling shifting syllable stress in the different grammatical forms of a word:




Added to this problem is the change in pronunciation of those syllables not taking the primary stress as the word changes form. Study the vowels sounds of the different syllables above as an example.
What to do to give student practice with syllable stress? Students do not learn by memorizing rules. Instead, they need structured activities that give them practice with syllable stress.
One activity involves the teacher preparing by writing down multi-syllabic words , with the primary stress at different positions, on separate cards or slips of paper. Students work in pairs or groups and categorize words by where the primary stress falls, on the first, second, third, or fourth syllable (such as in “potentiality”).
Another activity allows students to experiment with multi-syllabic words where the stress is placed on different syllables for each word. Students work in pairs or groups of three and are given cards on which one word is written two or three times, each instance indicating a different stressed syllable:

differently differently differently differently

Students will correct each other as they make forced mistakes and as they select the correct primary stress. Hearing and discussing incorrect stress placement is, I believe more beneficial than students simply being corrected when they make the primary stress placement incorrectly. This approach allows students to experiment with word stress and have fun with it. Not to mention that experimentation is an excellent form of practice and learning.

A third activity asks learners to practice shifting syllabic stress in different grammatical forms of a word:

informative information inform

possible possibility

drama dramatic dramatically

mortal immortality immortal

intelligence intelligently intelligentsia

Though the stress on two forms may be on the same syllable, it is useful for learners to discover this on their own, and to understand that the stress doesn't necessarily move all over the place as it may seem.
Finally, it is essential to help students understand that correct syllable stress cannot be overrated. Mistakes on syllable stress placement not only impede a non-native speaker's pronunciation of the word, but they disturb the rhythm of the English sentence. As a stress-times language, English relies on word and sentence rhythm for its comprehensibility as much as it does on the reduction of vowels in unstressed syllables.