Learning a language takes time and perseverance. If you are short of time, however, and you want to learn English grammar quickly, you can do so. The key is to listen to native speakers as much as possible and practice what you hear. You can learn basic English grammar in a relatively short time if you work hard at it.

Identify the grammar points that you want to learn. Do you need to brush up on your pronouns? Are idioms difficult for you to use? Choose one or two grammar points to learn about, and then focus on those. You will remember more if you only learn about one or two grammar applications and structures at a time.

Online videos and podcasts designed to help people learn English are helpful.
Listen to native speakers use the grammar points. Use online videos and podcasts aimed at English language learners, and that focus on the grammar you want to learn. Practice the dialogues you hear out loud, and then practice them out loud with a fluent English speaker. Listening to the grammar point being spoken will help you remember it better than if you only study it in a book.

Writing down what you learn will help you remember it.
Write a short story using the grammar structure you learned. You can write a short paragraph or two about yourself. Use the grammar you have learned throughout the story. Then read your story out loud to a friend who is a native English speaker. Your friend can then help you identify where you may have made mistakes. Rewrite your story and reread it until it's perfect.

Practice and review to retain what you have learned.
Practice using your English as much as you can. Make it a point to use the grammar you have learned every day, for a week or two. When you have mastered using the grammar you learned, move on and learn about a different aspect of English grammar.

Practice speaking sentences using the grammar you have learned.
Review what you have learned every month or so. Review it by reading articles that incorporate the grammar structures you have studied, by speaking with fluent English speakers, and by listening as much as possible to English that uses those structures. Write a short paper at the end of each month using the grammar points, too.