If从句如果较长，一般放在句子后面，否则给人头重脚轻的感觉。如本文有一句话：

原文： 如果您有问题需进一步了解，请直接与我联系。 原译：

If you have any questions regarding her or this recommendation, please contact me.

改译：

Please feel free to contact me directly if you have any further questions.

这个规律仅限于句子没有上下文或与上下文联系不紧密的情况（本例单独成段）。根据研究在有上下文的情况下，影响if从句位置的主要因素包括话题的连续性（topic continuity)、排比结构（parallelism)和if从句的限定性（qualification)等。

话题的连续性，是指旧信息和新信息在句子中交替出现，层层推进的情况。

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，句子重点，所以后置（这就意味着if从句前置）。

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表示强调，故后置；if从句所述内容，实际是对上一句内容的重复，属于旧信息，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 ).