It was only -------- she bagan to suffer that she realised t

先看后半句,she bagan to suffer that she realised the importance of health.-她开始遭受疾病困扰时才意识到健康的重要.
句子想要表达的意思是直到 xxx时候,才意识到.重要性.
所以只可能在when 和until 之间选.until 前面的主句动词只能用非瞬间性动词.如果想要表达直到才的意思,只能用not...until.
所以就用when了