Here are the ways stressed and unstressed syllables differ in American speech.
- Duration: stressed syllables are longer; unstressed are shorter. This forms a contrast in duration that is fundamental to native speaking
- Energy: stressed syllables have more energy or power that comes from the diaphragm usually; unstressed syllables are weaker, less energetic. This also forms a noticeable contrast between syllables.
- Pitch: stressed syllables often land on a higher pitch or tone than unstressed syllables. Another noticeable contrast.
- Clarity: stressed syllable vowels and consonants are usually more clearly articulated; they are enunciated more carefully, while unstressed syllables are often softer and less clear.
- Voicing: stressed syllable consonants are kept clearly articulated, but unstressed syllable consonants often dissolve into their voiced counterparts, e.g. T often is replaced by a D sound in unstressed locations. There are more that do this. You can watch my YouTube video about it, if it interests you: