However 何時是當副詞何時是當連接詞

He is nice; however, I don't like him. 這時的however是副詞還是連接詞, 那何時however當連接詞可否造幾個句子呢?

however有兩種身份adv. 及 conj. 但我們卻不可寫成下列的句子

He is nice, however, I don't like him.

如同 我們已知的but也是連接詞但我們卻可以寫成這樣

He is nice, but I don't like him.

為何不可寫成像but那樣呢?
Update: 謝謝你們的解說, 但我還是覺得不是很完整. 我想重點是: 它當副詞但卻有連接詞意味可是又不是連接詞,所以容易使人混亂.

而當連接詞時又不像and,but,or可以連接對等子句,片語或單字. 有點像after及before一樣是連接附屬子句(但我不知道這樣想對不對), 而書本上大多例句都差不多是副詞功能很少有連接詞的例子. 有誰知道可否告知
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