Rhythm is the word for the way stressed and unstressed syllables make patterns in speech. In sentences, we usually give more stress to nouns, ordinary verbs, adjectives and adverbs, and less stress-to pronouns, determiners, prepositions, conjunctions and auxiliary verbs.
She was SURE that the BACK of the CAR had been DAMAged.
Many linguists feel that the rhythm of spoken English is based on a regular pattern of stressed syllables. These follow each other at roughly regular intervals, and are pronounced more quickly and less clearly, and are fitted in between the regular stressed syllables. If several unstressed syllables come together, these are pronounced even more quickly so as not to disturb the rhythm. Compare the following two sentences. The second does not take much longer to say first although it has three more unstressed syllables, it has the same number of stressed syllables.
She KNEW the DOCTor.
She KNEW that there was a DOCTor.
Note, however, that this is a very complicated question, and not all experts agree about the way English rhythm works.