Complex sentence: Liverpool sat back for the majority of the first-half content to hold their advantage, but these tactics backfired when Celtic scored before half-time and in the second half to win 3--1 on aggregate.
Split into simpler sentences: Liverpool sat back for the majority of the first-half content to hold their advantage. However the tactics backfired when Celtic scored before half-time and in the second half to win 3--1 on aggregate.