English words with more than one syllable mostly have a fixed stress pattern. There are not many rules to show which syllable of a word will be stressed. One usually has to learn the stress pattern of a word along with its meaning, spelling and pronunciation.
Stressed on first syllable :
AFter, CAPItal, HAPpen, EXercise, EAsy
Stressed on second syllable :
InSTEAD, proNOUNCE, aGREEment, partiCULARly
Stressed on third syllable :
EnterTAIN, underSTAND, concernTRAtion
The stressed syllable of a word is the one that can carry an intonation movement. Many short phrases also have a fixed stress pattern.
frontDOOR, (not FRONTdoor)
LIVing room (not living ROOM)
Related words can have different stress patterns.
To inCREASE, an INcrease, PHOtograph, photoGRAPHer photoGRAphic