Elder 和 eldest 都可當形容詞和名詞用。Elder 用作形容詞時意為「(尤指家庭兩個成員中) 年紀較大的；較年長的」，與 older 同義。Eldest 用作形容詞時意為「(尤指家庭三個或三個以上的成員中) 年紀最大的；最年長的」，與 oldest 同義。Elder 和 eldest 只能用在名詞之前，亦即僅能當定語形容詞 (attributive adjectives) 用。例如：
- His elder sister has a beautiful face. (他姊姊有一張漂亮的臉蛋) (正)
His older sister has a beautiful face. (正) - My eldest daughter is in college. (我的長女正在上大學) (正)
My oldest daughter is in college. (正) - He’s my elder/eldest brother. (他是我哥哥/大哥) (正)
He’s my older/oldest brother. (正)
Elder 和 eldest 亦可當名詞用，前者意為「長者，長輩；(尤指家庭兩個成員中的) 年長者，年齡較大者」，而後者意為「(尤指家庭三個或三個以上的成員中) 年齡最大者；長子；長女」。例如：
- Of the two sisters Mary is the elder. (兩姊妹中瑪麗是姊姊) (正)
Of the two sisters Mary is the older. (正) － 從 older 前面有定冠詞可以得知，older 後面省略了名詞。 - John was the eldest of four brothers. (約翰是四個兄弟中的老大) (正)
John was the oldest of four brothers. (正) － 從 oldest 前面有定冠詞可以得知，oldest 後面省略了名詞。 - I am your elder by ten years. (我比你大十歲) (正)
I am your older by ten years. (誤) - Her eldest is at college. (她最大的小孩正在上大學) (正)
Her oldest is at college. (誤)
英美父子同名者所在多有，最顯著的例子就是美國總統唐納‧川普 (Donald Trump) 父子。由於川普在當總統之前就已因美國房地產大亨的身份而舉世聞名，所以他的兒子被固定叫做 Donald Trump Jr. (小唐納‧川普)，否則我們就可以稱川普為 Donald Trump Sr. 或 Donald Trump the elder (老唐納‧川普)。
Older 和 oldest 除了用作定語形容詞來修飾名詞外，亦可用作表語形容詞 (predicative adjectives)，放在連綴動詞的後面。此外，older 和 oldest 還分別意為「較舊的；較古老的」和「最舊的；最古老的」，而 elder 和 eldest 並沒有這樣的意思，亦即 elder 和 eldest 只能指人，不能指事物。例如：
- His wife is older than he (is). (他太太的年齡比他大) (正)
His wife is elder than he (is). (誤) - The museum is by far the oldest building in that country. (那座博物館是該國最古老的建築) (正)
The museum is by far the eldest building in that country. (誤)