Mastering English grammar is a long process that can take years for both native and non-native speakers of English. Though grammar mastery takes time, the more effort you put into it, the better you will get. Further, you can speed the learning process by doing a number of things.
Reading widely and closely will help you better understand grammatical structures and become used to accepted English word order. Read what you most enjoy reading, but also try to diversify your reading so that you can see how grammar functions in different genres. Newspaper articles, for instance, are usually more grammatically standardized than poetry. Try to pay attention to the ways the sentences in your reading are constructed. How often and when are commas used? In what contexts are certain verb tenses employed over others? Make notes about your grammar observations while you read.
Write every day to practice your grammar. Keep a journal where you record your thoughts or observations. Write out your entry for the day and then take time to go back over it, editing for grammar and punctuation. Practicing your writing on a regular basis will help your grammar to improve quickly. This is also a good way to measure your progress. Flip back to previous entries in your journal to see how much your grammar has improved over time.
Study Grammar Guides
Grammar guides are available in print books and online. These guides explain the grammatical rules from the basic to the complex and give examples of how these rules are employed in writing. Strunk and White's "The Elements of Style" is a classic grammar guide that gives simple rules about writing sentences. Grammar Girl is a more recent series of books and a website that address grammar in a clear, easy-to-understand way.
Get a Tutor
If you're a student, your school's Writing Center or Tutoring Center probably has tutors you can visit for free. If you aren't a student, tutoring companies and private tutors offer paid tutoring to individuals. You can find these tutors and companies online or through advertising on college bulletin boards or in college newspapers. A tutor can provide you with grammar instruction that is particularly suited to your writing and level of English.
Attend a Grammar or Introductory Writing Class
Colleges offer grammar classes to enrolled students and, through extension courses, also to members of the community. Many universities now offer grammar classes online that you can take from anywhere in the world. Introductory writing classes, such as first-year composition or developmental writing, will often provide grammar instruction as well, though the amount of instruction will vary by class and instructor.