[詞彙區別] but, however, nevertheless, nonetheless 的用法區別
1. but 是一個對等連接詞，即用來連接字、片語和獨立子句，表示前後所說的意義相反，或連接互相對比的兩個單位。
-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 前面需要加上逗號的原因是因為後面是完整的句子，或者為了讓句意清楚避免產生混淆，也會使用逗號。現代英文用法中，逗號基本上是說話者在長句中稍加停頓的地方。譬如：She is happy about moving away, but also a little sad. 但是若是句子不算長，而省略逗號並不會影響讀者的瞭解時，逗號省略是可以的 (She is sad but not shocked)。
◆but 的其他用法:
http://www.ldoceonline.com/dictionary/but_1
http://www.collinsdictionary.com/dictionary/english/but
http://www.merriam-webster.com/dictionary/but
◆對等連接詞:
http://www.taiwantestcentral.com/Grammar/Title.aspx?ID=61
http://www.chompchomp.com/terms/coordinatingconjunction.htm
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm
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2. however 可當副詞，當連接副詞時是用來轉折語氣。定義是"然而"，可以放句前。
however 的用法
https://bit.ly/3cC4kQR
http://www.ldoceonline.com/dictionary/however_1
http://www.taiwantestcentral.com/Grammar/Title.aspx?ID=119
◆而用 however 來轉折語氣時，可放句前但一定要用逗號 (,) 把它從句子其他部份分開。例句：This is a straightforward and effective method. However, it can be dangerous. (有些學者覺得用 However 開始另外一個句子是非正式的用法)。
◆不放逗號時，however 定義是無論如何，不管怎樣 (in whatever manner, to whatever extent, or no matter how: However carefully I explained, she still didn't understand.)。
◆至於 but 可否放在句前呢? 現在越來越多人這樣寫。老一輩的學者說不；但新一代的英文學者認為 but 用於句子開頭表達意思明確而直接，語氣反而比較強。
-It’s a smart and progressive plan. But do you think those changes go far enough?
http://www.quickanddirtytips.com/education/grammar/starting-a-sentence-with-however-right-or-wrong
http://blog.oxforddictionaries.com/2012/01/can-i-start-a-sentence-with-a-conjunction/
◆如果兩句句子意思緊接，你想用 however 卻不想把它分兩句，可以用分號 (；)。例句：This is a straightforward and effective method; however, it can be dangerous. 這是比較沒有爭議的寫法!
◆最後，however 可用於句子中間，在兩個逗號中出現，用作句子的停頓。甚至這樣子做可以加強 however 之後句子的語氣。
-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.
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv206.shtml
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3. nonetheless 等同 nevertheless 定義是"不過仍然"，比 however 的用法更為正式, 通常是用在寫作上。一些人會認為使用他們更會強調語氣。
◆Nevertheless 比較常使用在學術寫作出現的比率幾乎是 nonetheless 的一倍。
◆Nevertheless 用在主動的句子 (active voice) ， 而 nonetheless 比較常用在被動的句子 (passive voice). 。
Sources:
http://www.ldoceonline.com/dictionary/however_1
http://www.merriam-webster.com/dictionary/however
http://www.quickanddirtytips.com/education/grammar/starting-a-sentence-with-however-right-or-wrong
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv206.shtml
http://www.differencebetween.net/language/difference-between-nonetheless-and-nevertheless/ http://corpus.byu.edu/coca/
詞彙區別 Vocab Q&A: http://tinyurl.com/mzmvsns
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