How many times have you heard someone say, “I’m no good at grammar.”
Nonsense.
For one thing, even those English speakers who made bad grades in English class have already mastered 90% of the grammar they’ll ever need to know. They learned it with whatever English dialect they grew up speaking at home.
With a little effort, anyone can master the remaining 10% that enables employees and business owners to compete on an equal basis with professionals who have learned to speak a standard form of English.
Rock stars, certain types of comic actor, and professional athletes can get away with speaking non-standard English on all occasions, but people in other professions benefit from being able to speak and write a standard form of English when the occasion calls for it.
Language is a little bit like clothing; it makes a statement about the person using it.

Most of us like to wear jeans and tee-shirts, but we’d probably choose to wear something else for a television appearance on a show whose host is always dressed in suit and tie. When someone as wealthy as Adam Sandler shows up on a talk show dressed for mowing the lawn, we can be sure that his choice has something to do with promoting the uncultivated, boorish persona that he portrays in his movies.
The ability to speak and write a standard form of English is no more difficult than the ability to drive a car or play the piano. Not everyone will become race car drivers, concert pianists, or best-selling novelists, but anyone who is willing to learn a few rules and spend some time practicing can master the basics.