Sentence:  Over a relatively short period of time , usually about four to twelve weeks , the worker must <WSD>be</WSD> able to shift the focus , back and forth , between immediate external stressful exigencies ( `` precipitating stress '' ) and the key , emotionally relevant issues ( `` underlying problem '' ) which are , often in a dramatic preconscious breakthrough , reactivated by the crisis situation , and hence once again amenable to resolution . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)