but是完全可以放在句首的。

Invented Rules很多時候只是一些人的偏好，一種風格而已。作者說“For 250 years, grammarians have accused the best writers ofviolating rules like these, and for 250 years the best writers haveignored them.” 250年以來，優秀的作者們是不把這類規則當回事的。這裡就提到了but是否可以用在句首的問題：

牛津出版的Oxford Guide to Plain English中把“but不能放在句首使用”列為了6大寫作誤區（writing myths)之一，作者說：This is neither a rule of grammar nor even a widely observed convention.我們在閱讀和寫作的時候要注意觀察，然後知道什麼樣的規則是必須遵守的，什麼樣的規則是沒那麼重要的。在一個教雅思的網站上有人也討論了but是否可以放在句首的問題：

Can we begin a sentence with and or but?

This is a frequently asked question. Although some grammarians might argue that it is improper to begin a sentence with and or but, this practice has been in use since Anglo-Saxon times. In fact, starting a sentence with these conjunctions might create a powerful or dramatic effect.

However, you should consider avoiding an initial and or but in cases where the sentence will do just fine without the conjunction at the beginning. In some cases, the sentence beginning with and or but and the one that comes before it can be connected into a single compound sentence.

He soon got addicted to gambling. And that didn’t surprise anyone.

They abused him. They threw stones at him. But he didn’t protest.

Beginning a sentence with a conjunction is essentially a matter of convenience. It doesn’t make your sentences ungrammatical, but avoid this style if you are writing for an audience who might object to it.

這裡說but放在句首是可以的，但是要注意是為了強調的目的才這麼用的，不要濫用。Fowler's Modern English Usage中說“...unless contextual dislocation isbeing deliberately sought as a rhetorical device, it is not desirable to litter the pages with constuctions like He is tired. But he is happy.”

是的，but用於句首

but屬於連詞。連詞的作用就是連線兩個完整的句子，所以一般來說and和but前面都是逗號，在嚴謹的書面語中情況是不用在句首的（口語例外）

不過有時候介詞for可以和but搭配放在句首，意為：要不是，如果沒有。

例如：But for your help, I couldn't have finished the work in time.