correct my grammartical mistakes please. I really want to learn more about English ~There are my composition on below.

Followed by my previous reading, today I study the case studies. As I mentioned before, survey always have to examine a characteristic of many subjects for such as newspaper, newsroom, news syndicate and so on. However, In comparison with case studies, case studies examine many characteristic in a subject. That means one talk about macroscopic level, and another talk about microscopic level. Both of them are important in communication survey. On the other hand, for the part of reasoning about the data, it talks a lot about statistic. Some of them I heard before, so I familiar with them, such as median, variance, percentile. Moreover, there are a lot of proper-nouns. It makes me learn a lot. The most important of all, this article tell me far more important point telling me any resulting data must be managed through statistical methods. That is to say, to do a new survey, people should know the operation of statistic before.

  oh my god , you'r using big words ,, try to write it in simple words. as if you write it in your language , then you translate it by atranslator program.

see ya

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1. I highlight the corrections in red. 

2. Delete those in the  bracket ()

3. personal opinion,  defence welcomed.

Following my previous reading, today I talk about case studies. As I mentioned before, surveys always (have to) examine a characteristic of many subjects (for) such as newspaper, newsroom, news syndicate and so on. However, (In comparison with case studies,) case studies examine many characteristics in a subject. That means one talks at macroscopic level, while the other talks at microscopic level. Both of them are important in communication survey. (On the other hand), for the part of data analysis, survey talks a lot about statistic. Some of them I heard before, which I am familiar with (them), such as median, variance, percentile. Moreover, there are a lot of proper-nouns. It makes me learn a lot. (The) most important of all, this article tells me (far more important point telling me) that any resulting data must be managed through statistical methods. That is to say, to do a new survey, people should know the operation of statistic in advance.

Obviously something wrong about the text

sorry

It' s ok _{~i can see that ~}thank you very much `~

By the way, can you explain to me when should i use following by and followed by at the begining of my article?and what is the different between them ??

I’m still confuse how to use HAS or HAD ? Which one correct between “I HAS LEARNED ENGLISH TODAY or I HAD LEARNED ENGLISH TODAY”? Pls give me a clue. Thx

I have learned English today.

 Has is the third person form of Have.  Meaning "He HAS to" compared to "I HAVE to" 

While "had" is past tense, as speaking of something that happened in the past.

My understanding is that sometimes the difference in meaning could be rather subtle.

If I say "I have learned French", it may imply that I have learned the language and I still know it. But if I say "I had learned French", it may imply that though I had learned the language, I have now forgotten most of it.

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/upperf.htm

"I had cooked dinner when he arrived." means that I had cooked diiner before he arrived.

"I cooked dinner when he arrived." means that I started to cook dinner just after he had arrived.

http://web2.uvcs.uvic.ca/elc/StudyZone/410/grammar/pastpf.htm

Though Americans and Canadians generally say "learned" with "have/has" or "had", a lot of the British use the form "learnt".