1
the eldest + family member
作形容词时，常放在家庭称谓前。
She is the eldest daughter in the family.
她是家中最年长的女儿。
最年长的；常指在兄弟姐妹中排行第一的
先理解核心含义，再看它在真实句子里的用法。
最年长的；常指在兄弟姐妹中排行第一的
the oldest; most advanced in age, especially among siblings
She is the eldest daughter in the family.
她是家中最年长的女儿。
(玩家)从发牌者那里最先拿到牌
(of a player) Receiving cards from the dealer first, before any other players.
(手牌)拥有更高或更优的牌。
(of a hand) Having higher, or superior cards.
出生顺序中的第一个
first in order of birth
As the eldest daughter, she often helps take care of her younger siblings.
作为长女，她经常帮忙照顾弟弟妹妹。
The eldest son traditionally inherits the family business.
传统上，长子继承家族企业。
Among the three siblings, he is the eldest.
在三个兄弟姐妹中，他是最年长的。
作形容词时，常放在家庭称谓前。
She is the eldest daughter in the family.
她是家中最年长的女儿。
作名词时，指最年长的人，通常指兄弟姐妹。
He is the eldest of the three brothers.
他是三兄弟中最年长的。
常用'eldest of'结构表示群体中最年长者。
As the eldest of the children, she took responsibility.
作为孩子中最年长的，她承担了责任。
Not this
He is the oldest son in the family.
Use this
He is the eldest son in the family.
指家庭中兄弟姐妹排行时应用eldest；oldest常用于年龄或物体，不适用于家庭成员之间的长幼顺序。
Not this
She is the elder daughter.
Use this
She is the eldest daughter.
表示三个或以上子女中的最年长者需用最高级eldest；elder是比较级，仅用于两者之间的比较。
Not this
He is eldest son.
Use this
He is the eldest son.
最高级eldest前通常需要定冠词the，表示特指。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
长子
As the eldest son, he inherited the family estate.
作为长子，他继承了家族遗产。
长女
The eldest daughter often helps take care of her younger siblings.
长女经常帮忙照顾弟弟妹妹。
最大的孩子
Being the eldest child means having more responsibilities.
作为最大的孩子意味着有更多责任。
最年长者（通常指家庭中）
My sister is the eldest of three children.
我姐姐是三个孩子中最年长的。
最年长的兄弟姐妹
She is the eldest sibling and acts like a second mother.
她是最年长的兄弟姐妹，表现得像第二个母亲。
Use eldest naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
elder（年长的）加est就是最年长的eldest，专指兄弟姐妹中的老大。
在兄弟姐妹中排行第一的
Eldest is the superlative of old, used specifically for family members (e.g., eldest brother). As a noun, it is always preceded by 'the'.
Dictionary facts from ECDICT, WIKTIONARY.