Madrid entered the clash with a three-goal lead from the first leg, but came close to throwing their advantage away after a first-half double from Marco Reus.
The Spanish side held on to advance despite suffering a 2-0 defeat, and the Croatian has insisted that his side "need to learn" from the events in Germany.
"I don't know what happened," Modric told Marca.
"It may be that we were complacent because we won 3-0 and we started thinking it would be easy. I don't know. We need to learn from this match. We always say we need to learn but we never do."
The draw for the semi-finals takes place on Friday.