［情态动词］比较can 和be able to
<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>）， 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。例如：</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>的情况：</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><p> <span word="d">d</span>. 用于句首表示条件。</p><p> <span word="e">e</span>. 表示成功地做了某事时，用<span word="was">was</span>/<span word="were">were</span> <span word="able">able</span> <span word="to">to</span>，不能用<span word="could">could</span>.例如：</p><p> <span word="He">He</span> <span word="was">was</span> <span word="able">able</span> <span word="to">to</span> <span word="flee">flee</span> <span word="Europe">Europe</span> <span word="before">before</span> <span word="the">the</span> <span word="war">war</span> <span word="broke">broke</span> <span word="out">out</span>. = <span word="He">He</span> <span word="managed">managed</span> <span word="to">to</span> <span word="flee">flee</span> <span word="Europe">Europe</span> <span word="before">before</span> <span word="the">the</span> <span word="war">war</span> <span word="broke">broke</span> <span word="out">out</span>.</p><p> 他在战争爆发之前逃离欧洲。</p><p> 注意：<span word="could">could</span>有时不表示时态</p><p> 1）提出委婉的请求，（注意在回答中不可用<span word="could">could</span>）。例如：</p><p> <span word="Could">Could</span> <span word="I">I</span> <span word="have">have</span> <span word="the">the</span> <span word="television">television</span> <span word="on">on</span>？ 我能看电视吗？</p><p> <span word="Yes">Yes</span>， <span word="you">you</span> <span word="can">can</span>. / <span word="No">No</span>， <span word="you">you</span> <span word="cant">cant</span>. 可以/不可以。</p><p> 2）在否定句、疑问句中表示推测或怀疑。例如：</p><p> <span word="He">He</span> <span word="couldnt">couldnt</span> <span word="be">be</span> <span word="a">a</span> <span word="bad">bad</span> <span word="man">man</span>. 他不大可能是坏人。 </p><p> </p>
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