As soon as I realized there was a substitute on the stage, I was really disappointed.

substitute 是由前缀 sub-（表示"在...之下"或"替代"）和词根 stitute（来自拉丁语 statuere，意为"放置"）组成。这个词的字面意思是"被放在下面的"，引申为"替代者"或"替补"。在句中指替代主要演员上台表演的替补演员。