這三個形容詞都可用在名詞前面來表示數量，意為「足夠的」，如 We had adequate/enough/sufficient money for the journey. (我們為這趟旅行準備了足夠的錢)。不過，它們的語意仍有差別。Adequate 是表示某數量、時間、金錢等「足夠而已」，沒有一點多餘，甚至可能還差一點；enough 是表示超出所需之量的「非常足夠」，綽綽有餘；sufficient 則是表示「正好足夠所需之量」，甚至可能還多餘一點點。至於這三個字的否定詞或否定用法 (inadequate 或 not adequate/not enough/insufficient 或 not sufficient)，它們的語意差異似乎不明顯或不重要，均意為「不夠」、「不足」。
舉例而言，如果吃一碗牛肉麵需要 150 元，而你身上恰有 150 元，這是 adequate；若你有 200 元，則是 enough；若你帶了 160 或 170 元，那麼就是 sufficient。除上述的語意差異外，在複數名詞前面只能使用 enough 和 sufficient，不可用 adequate，如 Are there enough/sufficient bananas for everyone? (香蕉夠分給每個人嗎?)。在用法上，adequate 和 sufficient 都比 enough 來得正式一些。
綜上所述，茲舉數例如下供大家參考：
- The pay was barely adequate to support a family. (這份薪水僅夠養家活口)
- His income is inadequate to pay for it. (他的收入不夠支付此事的費用)
- We have enough chairs for everyone. (我們有足夠的椅子讓大家坐)
- There were not enough seats for all the guests. The supply of seats was quite inadequate. (座位不夠讓每個客人坐。座位數量很不夠)
- There was easily enough food for everyone. (食物非常足夠每個人吃)
- There was a sufficient amount of food. (食物正好足夠所需之量)
- Ten thousand dollars will be sufficient for the trip. (這趟旅行一萬元就夠了)
- Her income is not sufficient to support her family. (她的收入不夠養活家人)
- The food was insufficient for our needs. (食物不夠我們的需要)
- He has adequate/enough/sufficient money for his son to study abroad. (他的錢僅夠/非常夠/正好足夠他兒子出國唸書)