American English speakers divide longer sentences into intonation groups when they speak. In each intonation group they emphasize the focus word and then briefly pause before the next intonation group begins.
Monst non-native English speakers have never learned to use intonation groups when they speak English. For some people this causes their speech too sound too fast for American listeners to understand. For other people the lack of intonation groups causes their speech to sound very choppy and neutral to American listeners.
There are no solid rules for dividing longer sentences into intonation groups. It really depends on the idea that you are trying to convey. The best way to learn to use American English intonation is to sign up for accent reduction classes.
You can also learn how to use intonation groups and thought groups using the book Mastering the American Accent
This book includes four full length audio CDs that let you listen to and practice your intonation. The cost is less than $20.00.