Whether you're studying English in school currently or if you're an adult that needs to refresh your memory on certain English-related concepts, the best way is to look through various English textbooks and study guides. For example, if you struggle with subject-verb agreements or reading paragraphs for context clues, you can visit the library or bookstore for books that deal with those subjects. Many of these books also come with practice exercises.
Another good way to understand English properly is to read fiction books, non-fiction books, newspapers, scholarly journals, magazines and online articles at university websites. This reinforces the lessons you learned in your textbooks and study guides, and it also helps you generate ideas for writing exercises that can help you understand English better. For example, if you're in an English class and teacher asks you to write an informative essay, you may decide to write about American culinary traditions after reading some food magazines.
Since speaking proper English is just as important as writing it, practice speaking well in your daily conversations. For example, when calling a business, you should say, "Good morning, I'm calling in reference to the letter I received from you about..." Or when asking someone for an item, you should say "May I...?" instead of "Can I...?" If you're a student, consider joining the drama club, debate club or school newspaper because these groups will help you understand proper English.