The eldest person in a group is the one who was born first — the one who has been alive the longest.

The eldest person in a group is the one who was born first一群人裡的 eldest 是最早出生的那個人 the one who has been alive the longest也就是活得最久的那個人

English likes this word for brothers and sisters: the eldest daughter, his eldest son, the eldest of five. You can also say oldest, and many people do, for family and for everything else.

English likes this word for brothers and sisters英文很喜歡用這個字講兄弟姊妹 the eldest daughter, his eldest son, the eldest of five像 the eldest daughter、his eldest son、the eldest of five 這樣 You can also say oldest, and many people do你也可以說 oldest，很多人也這樣說 for family and for everything else用在家人身上、也用在其他所有事物上

Though actually the older, slightly more careful habit is eldest for people in a family and oldest for things and for mixed groups (the oldest building, the oldest student in the class). You almost never use eldest for a house or a book.

Though actually the older, slightly more careful habit is eldest for people in a family不過比較講究、比較舊的習慣是家人用 eldest and oldest for things and for mixed groups而東西和混合的群體用 oldest (the oldest building, the oldest student in the class)（像 the oldest building、the oldest student in the class） You almost never use eldest for a house or a book你幾乎不會用 eldest 來形容房子或書

It is not a verb. You usually say “the eldest” or “the eldest child,” not “an eldest” on its own.

It is not a verb它不是動詞 You usually say "the eldest" or "the eldest child," not "an eldest" on its own你通常會說『the eldest』或『the eldest child』，而不是單獨說『an eldest』

It is the “most old” form of old, sitting beside older and elder.

It is the "most old" form of old它是 old 的「最老」形式 sitting beside older and elder和 older、elder 並列在一起