How to assign sentence stress    

LEXICAL WORDS (content words, open-class items):
nouns, adjectives, adverbs, lexical verbs

STRUCTURAL WORDS (functional words, closed-class items):
prepositions, pronouns, auxiliary verbs

Sentence stress falls on the STRESSED SYLLABLES of all lexical words:

'sort, ob'jection, re'ceive, Mc'Cartney

Stuctural words may carry sentence-stress if emphasised in some way:

When cited: How do you spell "than"?
Comparative emphasis: "I don't think I can /kn/ do it"-"But I think you can /'kæn/ do it"

Polysyllabic structural words may carry sentence-stress:

un'til, be'hind

Adverbs are usually stressed, even if they are technically structural words:

She 'jumped 'off the 'table
It's 'too 'late to 'change our 'minds

Adverbs of position carry sentence stress:

I'm 'not 'coming 'here a'gain
I 'left it 'over 'there

Demonstratives are stressed:

'This 'book is 'really 'worth 'reading
'That's the 'stupidest i'dea I ve 'ever 'heard

Negatives are stressed:

I'm 'not 'coming 'here a'gain
We 'can t ex'pect 'everything
I 'hope you 'won t be an'noyed