Sentence:  The Frankfurt years <WSD>were</WSD> particularly noteworthy for his performance of Berg 's Wozzek soon after the Berlin premiere under Erich Kleiber , and the world premiere of Scho ^ nberg 's Von heute auf morgen . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)