How to master in English, especially in grammar? Please help me, people...
There really isn't a single or even a simple answer to that question. Different people learn in different ways, but generally learning a language takes time and a lot of practice. Also, there's much more to a language than the grammar. A grounding in grammar is very good, but that doesn't necessarily produce a fluent user. Practise and use your English at every opportunity, which is exactly what you can do here. With grammar, ask any specific question and someone will give you an answer. ;-)
If you are really serious about mastering English, then read all you can. Most students only want to study grammar and vocabulary, but reading is an excellent way to practice and reinforce what you have learned and to learn new vocabulary and grammar structures subconsciously, which approximates how native speakers learn their language.
If you try to read for 20 or 30 minutes every day, you will be amazed how rapidly your English will improve. After I finished my Spanish grammar courses (about 3 years), I began to study Spanish literature. At the end of the first year, my Spanish had improved more than in the previous two!
One word of caution, though. Don't just read one type of material because you will be limiting your exposure to different types of vocabulary and grammar structures. Try different types of writing: news, business, sports, science, humor, travel, cooking, etc. It is also important to choose readings that interest you. If you are not really interested in what you are reading, you will retain relatively little. The more interested you are in the content of the reading, the more you will retain. You can find a wealth of free material on the Internet.
Keep a notebook and jot down interesting words and phrases as you are reading. You can look them up later. Don't try to look up very much while you are reading. Try to understand as much as you can from the context and only look up words that are absolutely necessary. Don't worry if you don't understand a lot of things. Focus on the main ideas. With practice you will understand more and more, and what you have picked up will begin to show in your speech and writing.
Those interesting phrases (lexical chunks) I mentioned are especially useful for learning how to express yourself in English. Just as we learn to dance by watching someone who knows how, we also learn to speak and write by observation. Just ask a native speaker to compare his English before he began college with his English after he graduated!
If you do this conscientiously, your fluency and comprehension will increase by leaps and bounds.