Many words in spoken English are unstressed and this makes then hard to hear. In order to understand word stress, you need to know about content and function words.
• Content words are the big words: In general are stressed and they are fairly easy to hear. They are nouns, verbs, adjectives, and adverbs.
• Function words are the little words. In general are unstressed, and sometimes they can be very hard to hear. They are articles (a, an, the), pronouns (you, she), prepositions (in, on, at) , and helping verbs (have, is, does, did, will)