如果前後是兩個完整句子，通常是要加上逗號，使得句意清晰，但是如果兩句話很短的話，省略逗號也是可以的

But，連詞，表示"但是"，不需要用逗號和句子分開。舉例如下:

She is young but very experienced.

I'm sorry, but I disagree with you.

However，副詞，表示"然而"，要用逗號和句子隔開。

My room is small; however, it is comfortable.

My father, however, didn't agree.

However不能像but那樣直接連線兩個句子,這也是區別but的地方。

誤:

We all tried our best, however we lost the game.

正:

We all tried our best, but we lost the game.

We all tried our best; however, we lost the game.

We all tried our best. However, we lost the game.

but, 介詞，"除...以外"，用來排除同類中的一份子或從整體中除去一部分。

Nobody knew her but me.

Nothing but disaster would come from such a plan.