1. but 是一个对等连接词，即用来连接字、片语和独立子句，表示前后所说的意义相反，或连接互相对比的两个单位。
-It is expensive but useful.
-He enjoys rock climbing by himself, but not mountain hiking with others.
-It’s an old car, but I think we can fix it up.
◆ but 前面需要加上逗号的原因是因为后面是完整的句子，或者为了让句意清楚避免产生混淆，也会使用逗号。现代英文用法中，逗号基本上是说话者在长句中稍加停顿的地方。譬如：She is happy about moving away, but also a little sad. 但是若是句子不算长，而省略逗号并不会影响读者的了解时，逗号省略是可以的 (She is sad but not shocked)。
2. however 可当副词，当连接副词时是用来转折语气。定义是”然而”，可以放句前。
◆而用 however 来转折语气时，可放句前但一定要用逗号 (,) 把它从句子其他部份分开。例句：This is a straightforward and effective method. However, it can be dangerous. (有些学者觉得用 However 开始另外一个句子是非正式的用法)。
◆不放逗号时，however 定义是无论如何，不管怎样 (in whatever manner, to whatever extent, or no matter how: However carefully I explained, she still didn’t understand.)。
◆至于 but 可否放在句前呢? 现在越来越多人这样写。老一辈的学者说不；但新一代的英文学者认为 but 用于句子开头表达意思明确而直接，语气反而比较强。
-It’s a smart and progressive plan. But do you think those changes go far enough?
◆如果两句句子意思紧接，你想用 however 却不想把它分两句，可以用分号 (；)。例句：This is a straightforward and effective method; however, it can be dangerous. 这是比较没有争议的写法!
◆最后，however 可用于句子中间，在两个逗号中出现，用作句子的停顿。甚至这样子做可以加强 however 之后句子的语气。
-He was feeling tired. He went to work, however, and tried to concentrate.
-He was feeling tired. He, however, went to work and tried to concentrate.
3. nonetheless等同 nevertheless 定义是”不过仍然”，比 however 的用法更为正式, 通常是用在写作上。一些人会认为使用他们更会强调语气。
