if... then

if...与if...then在意义上并无区别。它们的区别仅仅表现在语体上。if...then主要用于正式的书面语体中，尤其是长句中，这样能使前后关系脉络分明。请考察下面的实例：

1. If $\mathrm{<span\; class="latin-run"><span\; class="latin-run">x</span></span>}=\mathrm{<span\; class="latin-run"><span\; class="latin-run">y</span></span>}$x = y and $\mathrm{<span\; class="latin-run"><span\; class="latin-run">y</span></span>}=2$y = 2 , then $\mathrm{<span\; class="latin-run"><span\; class="latin-run">x</span></span>}=2$x = 2 . (MED) 如果 $\mathrm{<span\; class="latin-run"><span\; class="latin-run">x</span></span>}=\mathrm{<span\; class="latin-run"><span\; class="latin-run">y</span></span>}$x = y ，而 $\mathrm{<span\; class="latin-run"><span\; class="latin-run">y</span></span>}=2$y = 2 ，则 $\mathrm{<span\; class="latin-run"><span\; class="latin-run">x</span></span>}=2$x = 2 。

2. If this year's harvest is good, then they will not need to import wheat. (CGEL) 如果今年的收成好的话，那他们就不必进口小麦了。

3. If you knew this all alone, then you could have told me. (CGEL) 如果只是你一个人知道这件事的话,那你可能已经告诉我了。

4. If Fache now realized he'd made a mistake, then it made perfect sense that he would tell Collet not to make a move. (Dan Brown, The Da Vinci Code) 如果法希现在意识到了错误, 那他让科莱停止行动就太有意义了。(《达·芬奇密码》)