为使大家更透彻地了解全文主旨句，请大家先仔细阅读下列句子，判断哪些句子是有效的主旨句，并说明理由：
1. Our classroom is clean， bright and cozy.
主旨过窄，无法展开来写，是无效的主旨。
2. Though facing many serious difficulties， our reform is still a successful one.
有效主旨，既不过于宽泛也不过于细节，留有继续拓展的余地。
3. The sun is very important.
这句话话题过大，无法把握，用几十万字的篇幅来叙述也未必说得够详尽。作为主旨显得过宽，比较含糊，不能让读者明白作者想写什么，因而是无效主旨。
根据英美人的写作习惯，全文主旨句在文章首段的最后一句出现为宜。全文主旨句和段落主题句是作者思维的起点，切题的准绳。好的段落主题句，不但切题，便于围绕主旨句进行扩展，而又常常给读者一种一针见血的感受。看到段落主题句，读者应大致了解段落要阐述的内容，因而段落主题句通常是一个语法结构完整、内容概括、用词简洁、明了的单句。通常将段落主题句置于段落的开头，可使文章结构更清晰，说服力更强。
4.参照提纲，紧扣主题句，完成段落。
有了段落主题句后，还需要顺着段落主题句的方向，参照提纲中的思路，从而完成各个段落。写作段落时，要尽可能多地搜集细节来支持主题。下面给考生呈上有关“电视对孩子不良影响”的引言段、各段主题句及结论部分，供大家欣赏参考：
Introduction： Television contributions to society， positive and negative， have been debated continually since this piece of technology invaded the average American household in the 1950s.
Topic sentences：
1. TV distorts reality by selecting certain kinds of images and omitting others and by portraying people in a stereotyped way.
2. TV affects human relationships as well as behavior by influencing our feelings about ourselves and our expectations for ourselves and others.
3. Exposure to stereotyped presentations can easily influence viewers' behavior toward unfamiliar people.
4. Perhaps most serious are the effects of information distortions on the child's self-image.
Conclusion：
TV programs should demonstrate a realistic lifestyle and a responsible sense of behavior. The many hours children spend in front of the television can obviously influence their view of the world. According to TV， how does the world in fact work ? As a society， we cannot afford the consequences resulting from the distortions of reality portrayed every day in these shows.
