I have a trouble in English writting

I have a trouble in English writting
How to distinguish the difference between the topic sentence, the topic and the central sentence. You can help me either in English or Chinese

Topic is a short phrase or a word which can conclude the content of the passage;
Topic sentence is one sentence that can help you determine the central idea of the paragraph or the passage;
Central sentence is the same thing as Topic sentence.
再问： So I am sorry to offend you,and one more question.What is the diffence between the topic sentence and the concluding sentence?
再答： When you write an essay, you need to have one topic sentence and one concluding sentence. The topic sentence is to tell the readers the main idea of this para; but concluding sentence is to conclude the para, to close this para to start a new topic. the concluding sentence is usually the last sentence.
再问： But I still cannot distinguish them, for can't the main idea conclude the para?I have no idea about it
再答： main idea is to tell the readers what you are talking about, but the conclusion is to close the para and start a new one. for excample, if you want to write an essay about the impacts of the Internet, you can talk about the personal privacy in one para. the topic sentence would be " the Internet has great impact on people's personal lives" and the concluding sentence would be "however, the Internet ont only brings the problems of personal privacy, but also some other nagative consequences " But i have to tell you. U dont need differentiate them, U just need to write neatly and logically. thats all. the names of topic&concluding sentence are to assist U to write in a systematic way. usually, topic sentence is the first sentence, and the concluding sentenve is the last. if U want, i can send U an essay as an model essay . Just tell me Ur email add.
再问： 3q very much, My e-mail address is drag0n9txh@vip.qq.com. But I really want to ask whether the concluding sentence must end with such pattern which the idea is different from the topic sentence or not.