Complex sentence: Since 1954, the majors have been played in their modern order (Masters, U.S. Open, Open Championship, PGA), except 1971, when the PGA was played prior to the Masters.
Split into simpler sentences: Since 1954, the majors have been played in their modern order (Masters, U.S. Open, Open Championship, PGA). The only exception since 1954 was 1971, when the PGA was played prior to the Masters.