托福独立写作作为一种学术写作测试，要求考生就某一话题给出合理的论点，并对每个论点都给出充分的论据。在教学中笔者发现，由于国内教育普遍忽视了对学生思辨能力的培养，加之知识面有限，许多学员在写作中或是笔头干涩，做不到well-developed；或是看似内容丰富，但条理混乱，忽视了well-organized；也有的自知不擅长说理，所以一味依赖举例，不但内容“发虚”，例子本身的质量也不易保证。这都严重影响了是否能拿下托福写作高分。本文就如何给出clearly appropriate explanations试做讲解。
以下题为例：
Do you agree or disagree with the following statement? It is better for children to grow up in a big city than in the countryside.
不难想到，对于孩子的成长，大城市的一大优势是优质教育资源。我们可给出如下主题句：
First and foremost, a big city provides children with more favorable educational condition.
众所周知，主体段接下来的论述必须紧密围绕主题句。而每个主题句必然包含两个要素：topic（话题）和controlling idea（中心思想，或者说针对话题的某个“角度”）。从主题句不难看出，这个段落显然是要从教育条件的角度来论述大城市的优势，而这一切的前提是教育的重要性：如果教育对于孩子无关紧要，那么接下来的所有论述就毫无必要了。因此，我们不难得到这样一条逻辑链：教育对孩子好——大城市的教育条件好——大城市对孩子好。也就是说，我们必须首先从controlling idea出发，证明教育对孩子的重要性。范文如下：
Education plays an essential role in our personal development since it helps one’s career success by offering abundant theoretical knowledge and practical skills; meanwhile, it also helps shape one’s personality to a large extent.
在此基础上，我们再将controlling idea和topic联系起来，论述大城市在教育条件方面的优势及其原因。范文如下：
Thanks to the urban economic prosperity and adequate financial support from the municipal government, schools in a major city are more likely to afford those expensive modern teaching facilities, e.g. well-equiped classrooms, labs and libraries. Moreover, the higher income and better life quality of the urban area attract more knowledgeable teachers to live and work there.
不难发现，这种以主体段主题句中的controlling idea为先导、topic为落脚点的说理论述不但逻辑严密，而且层层深入的写法也保证了字数，对于托福独立写作不失为一良策。
