Hi Khofifah!
I tell you about the short stories.
As you need to sell-out all of your products to the customers successfully, you must approach the customers as close as possible with your amazing marketing skills. You must PERSUADE the products to the customers such as the its qualities, its benefits, its functions. Therefore, you can manage and be successful to sell the products that you have.
If your question is "Do you think grammar is the most important thing to learn in English?"
Of course, it is important.
English grammar is the fundamental rules for someone who needs to understand English Language.
English grammar is similar to the marketing skills - PERSUADING skills to the customers from the story above.
As we have known, understood, and used the English grammar very well, we can master all language skills - LISTENING SKILLS, SPEAKING SKILLS, READING SKILLS, AND WRITING SKILLS.
Therefore, you can hold the world in your hand easily because the LANGUAGE USED IN THE WORLD IS ENGLISH LANGUAGE.
Furthermore, what you need to know BEFORE you master English Grammar is YOUR DETERMINATION AND INTEREST TO STUDY AND PRACTICE THE ENGLISH GRAMMAR.
FOR INFORMATION FOR US, English is NOT ONLY to learn BUT ALSO to practice.
Hope it enlighten all of you.
Cheers,