1
be an understudy to [actor]
名词用法，后接某人，表示“某人的预备演员、替角”。
He was an understudy to the famous actor.
他是一位著名演员的替补演员。
替角；预备演员
understudies — plural of understudy
先理解核心含义，再看它在真实句子里的用法。
替角；预备演员
an actor ready to replace another performer when needed
He was an understudy to the famous actor.
他是一位著名演员的替补演员。
替补演员；候补演员。
A performer who understudies; a standby for a study.
The lead actor lost his voice before the show, so the understudy took over the role.
主角演员在演出前失声了，所以替角接替了这个角色。
She has been asked to be the understudy for the main actress in the new play.
她被邀请担任新剧中女主角的替角。
Understudies often wait backstage in costume during the whole performance.
替角们常常穿着戏服在整个演出期间在后台等待。
名词用法，后接某人，表示“某人的预备演员、替角”。
He was an understudy to the famous actor.
他是一位著名演员的替补演员。
及物动词用法，直接跟角色名，指通过观察学习该角色以备随时顶替。
She understudied the role of the mother.
她为母亲一角担任替补演员。
不及物动词用法，后接 for 再接演员或角色，表示担任其替补。
She understudied for the lead actress during the tour.
巡演期间，她为女主角担任替补。
Not this
He is understudy for the lead actor.
Use this
He is an understudy for the lead actor.
understudy 表示“替角”时是可数名词单数，前面必须有不定冠词 an；中文没有冠词习惯，学习者容易漏掉。
Not this
She is an understudy of the leading role.
Use this
She is an understudy for the leading role.
understudy 表示“为某个角色/演员替补”时，固定搭配介词是 for（如 for the lead），不能按中文“的”直译成 of。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
担任替角
She acted as an understudy for the queen.
她担任女王的替角。
为某个角色当替角
He understudied the role of Hamlet.
他为哈姆雷特这个角色当替角。
以替角身份上场
The young actress went on as an understudy and impressed the audience.
那位年轻女演员以替角身份上场，给观众留下了深刻印象。
主角的替角
The understudy for the lead was ready to perform at any moment.
主角的替角随时准备好演出。
Use understudies naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
在主演之下(under)学习(study)角色，随时准备上台替补。
语域：neutral 常见场景：theatre/performance、rehearsal/backup、workplace learning 通常用于戏剧和表演领域，指替代正式演员的替补演员；作动词时多用于表示通过观察或实习来掌握某项工作。属于中性偏正式的说法，常见于剧院、排练或职业培训等场合。
这些词都以 -dy 结尾：study、buddy、muddy、ruddy、bloody 的结尾读 /di/，与 understudy 最后的 /stʌdi/ 在元音和后续辅音上相同；注意 sturdy 拼写相近，但读作 /ˈstɜːrdi/，并不押韵。
Dictionary facts from ECDICT, WIKTIONARY.