how to structure an essay:
首先，开头段（introduction）作为文章的起始部分，其作用包括引出话题内容(paraphrasing)，以及主题观点陈述(thesis statement)。关于thesis statement的写作方法，大家可以参考：http://www.baydue.com/news/faq/435.html
(注：Mini-outline—a short sentence briefly explaining all the key points the writer intends to discuss.)
然后，主体段落(body paragraphs)作为文章最重要的部分，不仅须要回应开头段中提到的主题观点，更要针对这些观点进行解释说理(explanation)或例证(exemplification)，即主体段的组成包括段落中心句(topic sentence)和(supporting details-explanation + exemplification)。
(注：Task 2要求—Give reasons for your answer and include any relevant example from your knowledge or experience.)
最后，结尾段(conclusion)虽然在重要性上略低于前两个部分(introduction + body paragraphs)，但是一个合理的总结可以更有效地支撑主题观点，回扣全文，因为总结中也需要重申主体段中的论点(restate main points)以及推导至段落中心句信息(topic sentence)。
以剑14 Test 2真题为例，进行结构分析：
Some people say that the main environmental problem of our time is the loss of particular species of plants and animals. Others say that there are more important environmental problems. Discuss both these views and give your own opinion.
Some people consider that the loss of individual species of plants and animals is the main environmental problem of our time. Attempts to preserve rare or dying species are carried out in zoos or other specialist plant and animal facilities, through breeding programmes, preservation orders and other forms of protection (paraphrasing-双方观点) and there was genius sadness among many when the last surviving white rhino died early this year (background/ hook).
Body paragraphs: 主体段落
Survival of the fittest is nature’s way of ruthlessly allowing weaker specimens to die out, although mankind has also contributed to failing animal numbers through hunting and poaching. (topic sentence) However, whether saving individual species should be the main focus of environmental protection is debatable. (explanation) Climate change and excessive use of plastic are just two major problems that deserve our attention. (transitional sentence-过渡句)
The evidence of climate change is there for us all to see: rising sea levels and associated flooding with the loss of homes at low land levels are becoming more frequent. (topic sentence) Many countries and island communities are now calling for urgent action to delay, halt or even reverse the process, although if we believe the experts, reversal is impossible. (explanation) Habitable areas are gradually shrinking, often for some of the poorest groups of people, but there seems to be little obvious action taking place. If the major contributors to climate change will not support the reduction of greenhouse gases, there is little hope for the future. (example)
Naturalists have also begun to expose the serious problems arising from discarded plastic waste. (topic sentence) Birds and animals can die gruesome deaths through being trapped or caught up in plastic bags or ropes, while the long process required for these plastics to decay means that they remain a threat for many years to come. (explanation) Worse, there is now beginning to be evidence of sea creatures ingesting smaller globules of disintegrating plastic, which harms them and all the other creatures in their particular food chain. (example)
The loss of particular species, both flora and fauna, is cause for regret, but I do not agree that they are the most important environmental problems these days. If climate change and damage from plastic continue (restate main points), there will be less and less food for everyone, including plants and animals.