Namely, English is considered a stressed language while many other languages are considered syllabic. What does that mean? It means that, in English, we give stress to certain words while other words are quickly spoken (some students say eaten!).
   Many speakers of syllabic languages don't understand why we quickly speak, or swallow, a number of words in a sentence. In syllabic languages each syllable has equal importance, and therefore equal time is needed. English however, spends more time on specific stressed words while quickly gliding over the other, less important, words.

单词重读的规律

①单音节词均重读。
例：beg[beg]，put[put]
   ②双音节词一般第一个音节重读。
例：doctor，better，apple。但是如有前缀a-,be-,re-, dis-, mis-, de-等双音节词，则第二个音节重读。

about
again
address
beside
report
mistake
decide
另外，双音节词的合成词，一般第一个音节重读。
例：blackboard，homework
但也有两音节均重读的：
′four′teen、′ un ′fair、′hel′lo、′up′stairs、 ′down′s tairs、 ′un′touched、 ′out′side、′Chi′nese、′nine′teen
③多音节词一般是倒数第三个音节重读。
'wonderful
im'mediately
 'interested
但双音节词加上前缀或后缀构成的多音节词，仍按原来的词根重音来读。
con′ductor
′carefully
dis′cover
as′sistant
④-ic，-tion，-sion等之前的音节要重读。
′question
ope′ration
compo′sition
′topic de ′cision
                              句子重音

定义：在美式英语连贯的说话中，不可能所有的词都同样的重要，必然有一些词较为关键，有一些则是次要一点的。一般来说，关键次要重读，这就是句子重音。而其他的词则不必重读。其中，名词、实义动词、形容词、副词、数词、指示代词、疑问词和感叹词等一般都要重读，而冠词、介词、连词和一般性的代词则不重读。

Sentence stress is the music of spoken English. Like word stress, sentence stress can help you to understand spoken English, especially when spoken fast.

Sentence stress is what gives English its rhythm or "beat". You remember that word stress is accent on one syllable within a word. Sentence stress is accent on certain words within a sentence.
Most sentences have two types of word:
content words
structure words

Content words are the key words of a sentence. They are the important words that carry the meaning or sense.

Structure words are not very important words. They are small, simple words that make the sentence correct grammatically. They give the sentence its correct form or "structure".

If you remove the structure words from a sentence, you will probably still understand the sentence.
If you remove the content words from a sentence, you will not understand the sentence. The sentence has no sense or meaning.

Why is this important for pronunciation?

It is important because it adds "music" to the language. It is the rhythm of the English language. It changes the speed at which we speak (and listen to) the language. The time between each stressed word is the same.

Rules for Sentence Stress in English

The basic rules of sentence stress are:
content words are stressed
structure words are unstressed
the time between stressed words is always the same
The following tables can help you decide which words are content words and which words are structure words:
Content words - stressed
Words carrying the meaning     Examples
main verbs                SELL, GIVE, EMPLOY
nouns    CAR, MUSIC, MARY
adjectives  RED, BIG, INTERESTING
adverbs  QUICKLY, LOUDLY, NEVER
negative auxiliaries  DON'T, AREN'T

Structure words - unstressed
Words for correct grammar  Example
pronouns     he, we, they
prepositions    on, at, into
articles     a, an, the
conjunctions   and, but, because
auxiliary verbs  do, be, have, can, must