The first question in using good grammar is “why?”
There are lots of reasons to use good grammar. The most important, in my opinion, is because of how people judge you based on the way you speak or write. We all may like to think that we do not judge a book by its cover, but we do. Those initial judgments are called first impressions.
The way you speak or write can convey laziness, disrespect, lack of intelligence, or rudeness if you do not use proper grammar. Using good grammar suggests that you are an intelligent, hard-working and respectful person who cares about how others see you.
Another reason for using good grammar, is to convey your message properly. Not only do you want to get your message across clearly, but you want to make sure that the correct message is received by your audience.
Writing that uses poor grammar and is full of errors is difficult to read. A reader can be so distracted by spelling and punctuation errors that the message gets lost. If there are too many errors, the reader may become frustrated, give up, and not bother to continue.
People are more likely to pay attention to you – and your message – if you are well-spoken and your message is well-written.