understudy造句
        The leading player is ill. Let his 
understudy to appear on the scene.
主角演员病了, 让他的替角出场.
The 
understudy was substituted when the leading actor fell ill.
主角演员患病,由替角代替演出.
The 
understudy is substituted when the leading actor falls ill.
“主角演员患病,由替角代替演出”.