i think u know the truth table of AND & OR logic
the same opperaton is going on here..
in first two statements..
PORTD |= (1<<PD0 ) and PORTD = (1<<PD0) both opperations are same
but in second statemnt(PORTD = (1<<PD0)), we r dng PD0 as a ONE and remaing all pins are ZERO.
in first statemnt (PORTD |= (1<<PD0)), only PD0 as ONE and we r not touching remng PINS..that mean
if PD1=1,
in first statemnt PDO=1 and PD1 also 1
but in second PD0 =1 and PD1 is ZERO.
and remaing also same…..
i hope u undersd…………