A substitution is when one thing or person is put in the place of another. The word covers both the act of swapping and, quite often, the thing that goes in — so 'the substitution took ten seconds' and 'this is a substitution for butter' are both normal.

A substitution is whensubstitution 指的是 one thing or person is put in the place of another一件東西或一個人被放到另一個的位置上。The word covers both the act of swapping這個字既包括替換這個動作 and, quite often, the thing that goes in也常常包括換上去的那個東西 — so 'the substitution took ten seconds'所以 'the substitution took ten seconds'（這次替換花了十秒） and 'this is a substitution for butter'和 'this is a substitution for butter'（這是奶油的替代品） are both normal都是正常的說法。

In sport it is the everyday word: when a tired player comes off and a fresh one comes on, that is a substitution, and coaches talk about 'making a substitution'. In cooking and shopping it means using a different thing because you cannot get the first one — if a shop sends you a different brand, they made a substitution. In school subjects, teachers use it for the act of putting a number in place of a letter: 'solve it by substitution'.

In sport在運動裡 it is the everyday word這是日常用字：when a tired player comes off and a fresh one comes on當一個累了的球員下場、換一個新的上場，that is a substitution這就是 substitution（換人），and coaches talk about 'making a substitution'教練會說 'making a substitution'（做換人調度）。In cooking and shopping在烹飪和購物裡 it means using a different thing because you cannot get the first one意思是因為拿不到原本那個東西而用別的代替 — if a shop sends you a different brand如果店家寄給你不同的牌子，they made a substitution他們就是做了一次 substitution（替代）。In school subjects在學校科目裡，teachers use it for the act of putting a number in place of a letter老師會用這個字指把數字放進字母的位置那個動作：'solve it by substitution'像是「用代入法解題」。

It sounds a bit formal or technical. In ordinary talk people usually say 'swap', 'change', or 'they put someone else in'.

It sounds a bit formal or technical這個字聽起來有點正式或專業。In ordinary talk在日常對話裡 people usually say 'swap', 'change', or 'they put someone else in'人們通常會說 'swap'、'change'，或「他們換了別人上」。

Notice that a substitution is meant to do the same job as the thing it replaced — that idea of 'doing the same job' is what separates it from just 'a change'.

Notice that要注意 a substitution is meant to do the same job as the thing it replacedsubstitution 應該要做跟被取代的東西一樣的工作 — that idea of 'doing the same job'這個「做一樣的工作」的概念 is what separates it from just 'a change'就是它跟單純的 'a change'（改變）不一樣的地方。