請教一下...

conj.
1. 但是
It is hot in summer here, but it is not cold in winter.
這裡夏天熱,但冬天不冷。
2. (通常用not...but...)而是
Our purpose is not to do it for you but to teach you to do it by yourself.
我們的目的不是替你做這事,而是教會你自己來做。
3. 可是,然而
She looks very young, but she is already in her 30's.
她看上去很年輕,可是她已三十多歲了。
4. (表示驚訝,不同意等)喔,哇
Heavens, but it snows.
天哪,真的下雪了。
5. (用來加強語句重複部分的語氣)一定
We will be here on time. But on time.
我們將準時到達這裡,一定準時。
6. (用來引入新話題)那就
But now to the last question.
現在就來談最後一個問題吧。
7. (常用於否定句)而不;若不
But that I saw it, I wouldn't have believed it.
若不是親眼目睹,我還不會相信呢。
It's not so high but we can jump it.
它並非高到我們跳不過去的地步。
8. (用於含doubt,question等字的否定句中,相當於that)對於
There is no doubt but he won.
對於他獲勝這一點沒有疑問。
prep.
1. (用於nothing,nobody,who,all等詞後面)除...以外
No one saw it but me.
除了我沒有人看到。
Dick seldom talks of anything but music.
狄克很少談及音樂以外的事情。
ad.
1. 只,僅僅,才
It took her but a few days to learn it.
只不過幾天時間她就學會了。
2. 【美】【俚】(用來加強語氣)肯定地,絕對地
Go there but fast!
趕快到那兒去!
pron.
1. (關係代名詞,相當於who...not,which...not)沒有...不
Not a man but felt it.
沒有人不感覺到它。
n.
1. 用"但是"來表示反對;異議[C]
Not so many buts, please.
請別老是反對個沒完。

But可否置於句子的開始？

其實，是可以的，但是就不太禮貌。適用於寫信給你的親人或朋友。

當然，把but置於句子中間是最好的。