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
Example
main verbs
SELL, GIVE, EMPLOY
nouns
CAR, MUSIC, MARY
adjectives
RED, BIG, INTERESTING
adverbs
QUICKLY, LOUDLY, NEVER
negative auxiliaries
DON'T, AREN'T, CAN'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
Exceptions
The above rules are for for what is called "neutral" or normal stress. But sometimes we can stress a word that would normally be only a structure word, for example to correct information. Look at the following dialogue:
"They've been to Mongolia, haven't they?""No, THEY haven't, but WE have.
Note also that when "be" is used as a main verb, it is usually unstressed (even though in this case it is a content word).