高中英语词汇用法比较：have to与must，can与be able to
<p>比较<span word="have">have</span> <span word="to">to</span>和<span word="must">must</span>的用法</p><p>1) 两词都是必须的意思，<span word="have">have</span> <span word="to">to</span> 表示客观的需要， <span word="must">must</span> 表示说话人主观上的看法，既主观上的必要。</p><p><span word="My">My</span> <span word="brother">brother</span> <span word="was">was</span> <span word="very">very</span> <span word="ill">ill</span>, <span word="so">so</span> <span word="I">I</span> <span word="had">had</span> <span word="to">to</span> <span word="call">call</span> <span word="the">the</span> <span word="doctor">doctor</span> <span word="in">in</span> <span word="the">the</span> <span word="middle">middle</span> <span word="of">of</span> <span word="the">the</span> <span word="night">night</span>. 我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)</p><p><span word="He">He</span> <span word="said">said</span> <span word="that">that</span> <span word="they">they</span> <span word="must">must</span> <span word="work">work</span> <span word="hard">hard</span>. 他说他们必须努力工作。（主观上要做这件事）</p><p>2) <span word="have">have</span> <span word="to">to</span>有人称、数、时态的变化，而<span word="must">must</span>只有一种形式。但<span word="must">must</span> 可用于间接引语中表示过去的必要或义务。</p><p><span word="He">He</span> <span word="had">had</span> <span word="to">to</span> <span word="look">look</span> <span word="after">after</span> <span word="his">his</span> <span word="sister">sister</span> <span word="yesterday">yesterday</span>.</p><p>3） 在否定结构中： <span word="dont">dont</span> <span word="have">have</span> <span word="to">to</span> 表示不必</p><p><span word="mustnt">mustnt</span> 表示禁止，</p><p><span word="You">You</span> <span word="dont">dont</span> <span word="have">have</span> <span word="to">to</span> <span word="tell">tell</span> <span word="him">him</span> <span word="about">about</span> <span word="it">it</span>. 你不一定要把此事告诉他。</p><p><span word="You">You</span> <span word="mustnt">mustnt</span> <span word="tell">tell</span> <span word="him">him</span> <span word="about">about</span> <span word="it">it</span>. 你一定不要把这件事告诉他。</p><p>比较<span word="can">can</span> 和<span word="be">be</span> <span word="able">able</span> <span word="to">to</span></p><p>1）<span word="can">can</span> <span word="could">could</span> 表示能力；可能 (过去时用<span word="could">could</span>),</p><p>只用于现在式和过去式(<span word="could">could</span>)。<span word="be">be</span> <span word="able">able</span> <span word="to">to</span>可以用于各种时态。</p><p><span word="They">They</span> <span word="will">will</span> <span word="be">be</span> <span word="able">able</span> <span word="to">to</span> <span word="tell">tell</span> <span word="you">you</span> <span word="the">the</span> <span word="news">news</span> <span word="soon">soon</span>. 他很快就能告诉你消息了。</p><p>2）只用<span word="be">be</span> <span word="able">able</span> <span word="to">to</span></p><p><span word="a">a</span>. 位于助动词后。</p><p><span word="b">b</span>. 情态动词后。</p><p><span word="c">c</span>. 表示过去某时刻动作时。</p>
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