however 等於 but 嗎？

很多人對 but 和 however 有個誤解，以為它們的差別是 however 比 but 相對 formal，又或者 however 是書面語、but 是口語。

事實上並非如此，即使 however 沒有 but 那麼口語，也不至於是 formal 的用詞。

兩個詞的差別在於詞性（part of speech），有太多人誤以為兩個都是連接詞（conjunction），其實 but 才是連接詞，however 是副詞（adverb），而詞性上的差別會影響到子句結構（clause structure）。

舉個例子：
1) He said he was telling the truth.
2) The police did not believe him.

上面兩句如果用 but 連接會寫成：
He said he was telling the truth, but the police did not believe him.

不過，以上這句如果寫 He said he was telling the truth, however the police did not believe him 是不對的，因為 however 不是連接詞，不可以就這樣連接兩句。

however 是一個副詞，你可以當它好像 actually、certainly、hopefully、unfortunately 等，這些詞又稱為 sentence adverb（句子副詞），在結構上要開新一句寫，而且後面要加 comma，例如：
He said he was telling the truth. However, the police did not believe him.

因為 however 是副詞，所以它的位置比較彈性，上句的 however 可以搬到句子的中間或結尾：
He said he was telling the truth. The police, however, did not believe him.
He said he was telling the truth. The police did not believe him, however.