stress placement in english

hi, i have a phonology test tomorrow. the subject is "stress placement and justification". but i have no idea how to justify the stress placement. for example, in the word "presidency" stress is on the first syllable. but in the word "refuse" stress is on the 2nd syllable. i know its like that because it sounds better, but i cant justify it.
does anyone have a rule or a tip to help me ? thks

>>1
They're asking you to explain why English words are stressed as they are? Good luck. Next up: rationale for the assignments of genders to nouns in German.