The penult, if short and accented, must have the acute.
If long and accented, the penult has the acute if U is long, the circumflex if U is short. This variation depends on the rule of contonation and mora.
Short P – length of U does not matter:
πόνον – short P, short U
πόνων – short P, long U
Long P – length of U determines accent:
δῶρον – long P, short U
δώροις – long P, long U