N. V. Tatsenko introduction to theoretical phonetics of english

Function words usually have strong forms when they are:

1. 
   at the end of the sentence, e. g. What are you looking at? Where are you from?
   
2. 
   used for emphasis, e. g. Do you want this one? No. Well, which one do you want? That one.
   
3. 
   used for contrast, He is working so hard. She is but not he.
   

In ordinary, rapid speech such words can occur much more frequently in their weak form than in their strong form.
The main function of sentence stress is to single out the focus/the communicative centre of the sentence which introduces new information.
Sentence Focus. Within a sentence/an intonation unit, there may be several words receiving sentence stress but only one main idea or prominent element. Speakers choose what information they want to highlight in an utterance/sentence. The stressed word in a given sentence which the speaker wishes to highlight receives prominence and is referred to as the (information) focus/the semantic center.
When a conversation begins, the focus/the semantic center
is usually on the last content word, e. g. Give me a HELP. What’s the MATTER? What are you DOING?
Words in a sentence can express new information (i. e. something mentioned for the first time (rheme)) or old information (i. e. something mentioned or referred to before (theme)). Within an intonation unit, words expressing old or given information (i. e. semantically predictable information) are unstressed and are spoken with lower pitch, whereas words expressing new information are spoken with strong stress and higher pitch. Here is an example of how prominence marks new versus old information. Capital letters signal new information (strong stress and high pitch):

1. 
   I've lost my HAT.
   
2. 
   What KIND of hat?
   

1. 
   It was a SUN hat.
   
2. 
   What COLOR sun hat?
   

1. 
   It was YELLOW. Yellow with STRIPES.
   
2. 
   There was a yellow hat with stripes in the CAR.