點解「mass」讀「mix」，但係「massive」就讀「ma sive」?

Mass is not read as mix, but m-ass. Massive is read is Mes-sive. Any word with -sive at the back, the syllable "a" will be read as "e".

a有2個音-a音.亞音;
mass,媽s
massive,(mei)sive