台灣人口語常犯的錯誤在於 must, need, should, may 這些情態助動詞 (modal verbs) 使用不恰當，會讓人覺得你很無禮 (rude) ，比如說，外國朋友跟你說他讀書/工作做不完，你建議他 You need to work hard all night. 保證一秒惹毛外國朋友。
首先，我們來了解情態助動詞 (modal Verbs) 是用來表達可能性、需要性、和必須性。多數人知道 “can 的過去式是 could”, “will 的過去式是 would”，”may 的過去式是 might”，但助動詞並沒有你想像中的簡單，它是具有情緒程度上不同的意思。當你給朋友建議時，”need” 是強烈命令性的語氣，而 “should” 和 “will” 是中性的建議語氣。而 would better 是很委婉的提議。回到開始的例子，更好的說法是 You’d better work hard all night.
然而，情態助動詞 (modal Verbs) 並不只用來給建議表達某件事的需要性，它也可用在某件事發生的機率。例如：You must be very tired. 這裡 must 不是命令，而是依邏輯推斷。以下整理情態助動詞的用法：
情態助動詞
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表達
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例句
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Must
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強烈的要求
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You must stop when the traffic lights turn red.
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邏輯推斷
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He must be very tired. He's been working all day long.
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Must not
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禁止
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You must not smoke in the hospital.
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Need
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需要 (較強烈)
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you need to let it go.
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need not
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不需要
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I need not buy tomatoes. There are plenty of tomatoes in the fridge.
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Should
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建議 (較強烈)
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You should (ought to) take an umbrella in case it rains.
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Ought to
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建議 (較 should弱)
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Can
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能力
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I can swim.
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許可(較直接)
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Can I use your phone please?
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可能(較高機率)
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Smoking can cause cancer.
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Could
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能力 (過去式)
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When I was younger I could run fast.
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許可(較委婉)
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Excuse me, could I just say something?
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可能(較低機率)
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It could rain tomorrow!
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May
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許可(較委婉)
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May I use your phone please?
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可能(較低機率)
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It may rain tomorrow!
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Might
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許可(更委婉)
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Might I suggest an idea?
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可能(更低機率)
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I might go on holiday to Australia next year.
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