If從句如果較長(zhǎng)，一般放在句子后面，否則給人頭重腳輕的感覺(jué)。如本文有一句話(huà)：
原文： 如果您有問(wèn)題需進(jìn)一步了解，請(qǐng)直接與我聯(lián)系。 原譯：
If you have any questions regarding her or this recommendation, please contact me.
改譯：
Please feel free to contact me directly if you have any further questions.
這個(gè)規(guī)律僅限于句子沒(méi)有上下文或與上下文聯(lián)系不緊密的情況（本例單獨(dú)成段）。根據(jù)研究在有上下文的情況下，影響if從句位置的主要因素包括話(huà)題的連續(xù)性（topic continuity)、排比結(jié)構(gòu)（parallelism)和if從句的限定性（qualification)等。
話(huà)題的連續(xù)性，是指舊信息和新信息在句子中交替出現(xiàn)，層層推進(jìn)的情況。
If higher temperatures persist after the 21st century it could result in very large impacts. For instance, the large sea-level rise that would result from the melting of the Greenland and Antarctic ice sheets would have major repercussions on coastal areas.
it could result in very large impacts是新信息，句子重點(diǎn)，所以后置（這就意味著if從句前置）。
Warming and sea level rise caused by human activities will continue for centuries, even if greenhouse gas concentrations were to be stabilized. If warming persists over many centuries，it could lead to a complete melting of the Greenland Ice sheet, increasing global sea levels by about 7m.
Even if表示強(qiáng)調(diào)，故后置；if從句所述內(nèi)容，實(shí)際是對(duì)上一句內(nèi)容的重復(fù)，屬于舊信息，故前置。
In 1998，46 million people live in areas at risk of flooding (BBC News，1997). This amountcould increase rapidly if sea levels rose. Scientists estimate that a sea level rise of only 50 centimetres would increase the number of people at risk to 92 million. A sea level rise of 1 metre would put 118 million people at risk. Scientists believe that there will be a sea level rise of 50 centimetres over the next 40 to 100 years (BBC News，1997 ).
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