1. Why should we learn English
1 studying English enables you to watch American movies, read English books and listen to English songs.
2 as English is an international language, you will be able to communicate with foreigners when you are on a trip abroad. Traveling will be more interesting that way.
3 It is a good idea to make friends with foreigners.
4 it will be fun and it will expand your view of the world. you can also become familiar with the customs and habits of different cultures.
Briefly said, English is so useful to us that we should all learn it. 

2. Dear xiaodong,
I am sorry to hear that you are having trouble in English listening .However, it is easy to change this situation and I am glad to give you some suggestions.
Firstly. Why not listen to English songs ? It is an easy way to learn some idiomatic expressions ,which is useful in your English listening.
Secondly, it would be a good idea if you watch English films ,or watch news on CCTV9. In that way, you can practice your understanding of English.
Thirdly,you should take chances to listen to native speakers. And you should often go to English corners.
I am sure you will make progress by taking advice above.
Yours,
Xiao Ming

3. 1 Knowledge refers mainly to grammar rules and vocabulary.
2 avoid reading anything that you know is too difficult for you. Reading should be a pleasure
3 If you read enough, you will find that you will acquire a large vocabulary without needing to turn to the dictionary all the time.
4You should read as widely as you can.
5The best way to give up smoking is to stop smoking cigarettes.The best way to read an article is to read it.
6. Keeping a diary is an excellent way of improving your written English

4. 7 It's totally meaningless learning grammar for its own sake.
8 Any communicative or practical use of English is better than studying grammar for its own sake.
9 Sentence structure is the way we show the relationship between ideas and events.
10 Reading a newspaper is the best thing you can do to improve your English.
11 You have to discount your native language