Sentence:  But Schnabel <WSD>was</WSD> a great teacher in addition to being a great performer , and the fact that four of the ten versions I listened to are by Schnabel pupils ( Clifford Curzon , Frank Glazer , Adrian Aeschbacher , and Victor Babin ) also sheds light on the master 's pedagogical skills . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)