This amount (指前面提到的人数）属于旧信息，故前置；if sea levels rose是新信息，故后置，正好与后面的a sea rise of only 50 centimetres相衔接。
在if-then这样的结构中，如果有两个以上的if形成排比，就把if从句后置；如果有两个以上的then,就把then后置：
The potential for runaway greenhouse wanning is real and has never been more present. The most dangerous climate changes may still be avoided if we transform our hydrocarbon based energy systems and if we initiate rational and adequately financed adaptation programmes to forestall disasters and migrations at unprecedented scales. The tools are available, but they must be applied immediately and aggressively.
再如：
If the introductory language for the list is an incomplete sentence —
• End the introduction with a dash；
• End each item in the list except the last item with a semicolon ；
• After the semicolon in the next-to-last item in the list, write“and”or “or”as appropriate； and
• End the last item in the list with a period.
所谓if从句的限定性，是指if从句起到补充说明的作用，例如：
[Carl is trying to save a cat，who climbed onto a pole and could not come down. ] “Catch me if I fall，Emil”，he called back as he began his ascent.
这里的if I fall并非句子重点，但放在前面并不自然，因为它的主要作用是补充说明。
影响if从句位置的因素还有其他，需要通过观察和体会，逐渐把握。
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