Rules for Sentence Stress in English

The basic rules of sentence stress are 

content words are stressed

1. structure words are unstressed
2. 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)