No matter what language you use to speak or write, using correct grammar not only helps you communicate more effectively and precisely, but also helps you avoid embarrassment. Around the world, correct grammar is an indication that the speaker or writer is an educated person who understands the nuances of the language, while grammar errors can indicate that you are not focusing on your words or, worse, that you do not understand the mechanics of your own language.
Incorrect grammar can often lead to sentences that mean little or nothing. Native speakers rarely commit errors this gross when speaking, but non-native speakers often make errors that render their sentences incomprehensible. For example, an attempt to tell a taxi driver that you want to go to the mall might come out as "I wanted going mall," which means nothing (though a talented driver could figure out what the speaker is trying to say). The same happens when English-speakers try to speak in other languages whose grammar is unfamiliar.
Communicating the Correct Meaning
Some grammar errors lead to sentences that mean something--just not what you intended. For example, misplacing the modifier "only" could lead to this sentence: "I only drive to work and back." If you meant that you never walk to work, nor take a bus, great. But if you meant that the only route you ever drive is the one between your house and your office, and you never drive anywhere else, then you should have said "I drive only to work and back."
Making an Impression
Even if your grammar is good enough to make others understand what you mean, constant errors might give them the impression that you are not highly educated or that you are not paying attention to what you are saying or writing. Even if they otherwise would think highly of your words, your errors might simply distract them. Good grammar keeps your readers or listeners focused on what you have to say, not on how you are saying it or why you are making mistakes.
Standard Written English
English is a constantly evolving language with many dialects. Spoken English can sound very different from region to region, even within the same English-speaking country.
However, over the years, English writers and educators have come to a consensus about most aspects of English grammar, establishing what is known as "Standard English" or "Standard Written English." This is the form of English considered "correct" for the purposes of formal writing and speaking.
How to Improve Your Grammar
The quickest way to learn correct grammar is to absorb it as a child, from hearing other people speak the language correctly.
For adults, two possibilities include reading and taking classes. Reading books, articles and websites in Standard English can help you gain a "feeling" for the flow of the language. Grammar courses and exercises will teach you why certain constructions are incorrect and how to fix them. Doing grammar exercises can help you internalize the lessons. Many community colleges offer English writing courses, and you can also find a variety of grammar websites on the internet, many of which include quizzes and other interactive learning aids.