一个段落的主题思想靠推展句来实现，如果只有主题句而没有推展句来进一步交待和充实，就不能构成一个完整的段落。同样，虽然有推展句，但主题思想没有得到相对圆满的交待，给读者一种意犹未尽的感觉。这样的段落也不能完成其交际功能。例如：

　　Physical work can be a usul form of therapy for a mind in turmoil. Work concentrates your thoughts on a concrete task. Besides, it is more usul to work--you produce something rather than more anxiety or depression.

　　本段的主题句是段首句。本段的两个推展句均不能回答主题句中提出的问题。什么是“a mind in turmoil”(心境不平静)Physical work又如何能改变这种情况?为什么它能起therapy的作用?读者得不到明确的答案。

　　若要达到完整就必须尽可能地简明。例如：

　　It is not always true that a good picture is worth a thousand words. Often writing is much clearer than a picture. It is sometimes difficult to figure out what a picture means, but a carul writer can almost always explain it.

　　段首句所表达的主题思想是一种看法，必须有具体事例加以验证。上述两个推展句只是在文字上对主题作些解释，整个段落内容空洞，简而不明。如果用一两个具体的例子的话，就可以把主题解释清楚了。比如下段：

　　It is not always true that a picture is worth a thousand words. Sometimes, pictures are pretty useless things. If you can't swim and fall in the river and start gulping water, will you be better off to hold up a picture of yourself drowning, or start screaming "Help"?

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