Today—the day after federal income taxes are payable—is National Stress Awareness Day. (来源：英语学习门户网站EnglishCN.com)

How to be aware of stress in our language? Trust us when we say that investigating stress in English speech is enough to make heads ache.

For starters, English is considered a stress-timed language. That is, stressed or accented syllables appear relatively predictably, at a constant rate, and non-stressed syllables are shortened to accommodate the stress. Unstressed vowels are also known as neutral vowels; the schwa, the upside-down lower case E, represents the phonetic sound of various unstressed, shortened vowels.

If you've got all that, you're ready to move on to stressed vowels. When they're stressed, short vowels can become diphthongs. What's a diphthong? A rough translation of the Greek ancestor of diphthong is "two sounds;" in English, the diphthong names the gliding monosyllabic speech sound that starts at or near the articulatory position for one vowel and moves to or toward the position of another. The vowel sound in toy is a diphthong, for example, and so is the vowel sound in wise.