The underside of something is its bottom surface — the face that points at the ground, or the face you cannot see from where you normally stand. The underside of a leaf, the underside of a car, the underside of a plate, the underside of a bridge. It takes 'the' almost every time, and 'of' follows it naturally, because an underside always belongs to something.

The underside of something某樣東西的underside is its bottom surface就是它的底面 — the face that points at the ground朝著地面的那一面, or the face you cannot see from where you normally stand或是你平常站的位置看不到的那一面. The underside of a leaf葉子的底面, the underside of a car車子的底面, the underside of a plate盤子的底面, the underside of a bridge橋的底面. It takes 'the' almost every time幾乎每次前面都會加「the」, and 'of' follows it naturally後面自然會接「of」, because an underside always belongs to something因為underside一定是屬於某樣東西的.

Notice how it differs from 'bottom': the bottom of a box is the part it rests on, which you might think of from inside or outside, but underside is specifically the outer face seen from below — the view an insect on the floor would have. That is why so many uses involve looking, checking, hiding: you turn a leaf over to check the underside for insects, mechanics look at the underside of a car for rust, and somebody tapes an envelope to the underside of a drawer.

Notice how it differs from 'bottom'注意它跟「bottom」不一樣: the bottom of a box is the part it rests on箱子的bottom是它靠著放的那個部分, which you might think of from inside or outside可以從裡面或外面去想, but underside is specifically the outer face seen from below但underside專指從下面看到的外側那一面 — the view an insect on the floor would have就是地板上一隻蟲會看到的那個視角. That is why so many uses involve looking, checking, hiding這就是為什麼很多用法都跟查看、檢查、藏東西有關: you turn a leaf over to check the underside for insects你把葉子翻過來檢查底面有沒有蟲, mechanics look at the underside of a car for rust技師看車子的底盤面有沒有生鏽, and somebody tapes an envelope to the underside of a drawer有人會把信封貼在抽屜的底面.

English also uses it figuratively for the hidden, uglier part of something otherwise pleasant: "the underside of the city's economic miracle" means the poverty and hardship underneath the success story. That figurative use is a bit literary but common in serious writing.

English also uses it figuratively for the hidden, uglier part of something otherwise pleasant英文也會用它來比喻某個原本美好的事物裡藏著的醜陋一面: "the underside of the city's economic miracle"「the underside of the city's economic miracle」 means the poverty and hardship underneath the success story指的是成功故事底下的貧窮和苦難. That figurative use這種比喻用法 is a bit literary有點文學性 but common in serious writing但在正式文章裡很常見.