As a general rule,

▪ Stressed words in a sentence are considered content words. These include nouns (e.g. kitchen, Peter), (most) main verbs (e.g. visit, construct), adjectives (e.g. beautiful, interesting), and adverbs (e.g. often, carefully).

▪ Non-stressed words are considered function words. These include determiners (e.g.: the, a, an), auxiliary verbs (e.g.: is, am, were), prepositions (e.g.: before, of, in), conjunctions (e.g.: but, and, so), and pronouns (e.g.: they, she, he)

Read the following sentence aloud:
The beautiful mountain appeared transfixed in the distance.

Now read this sentence aloud:
He can come on Saturdays as long as he doesn’t have to do any homework in the evening.

Notice that the first sentence actually takes about the same time to speak well! Even though the second sentence is approximately 30% longer than the first, the sentences take the same time to speak. This is because there are five stressed words in each sentence.