Grammar is a subject too complex to be summarized here. Poor grammar may do nothing worse than irritating your readers. Sometimes, however, poor grammar can make your writing confusing or impossible to understand.
Be careful with spelling, and especially with homonyms (words which sound the same but are spelled differently). You may have correctly spelled a word that you didn’t mean to use. “Joe is a little horse” is a very different statement from “Joe is a little hoarse.”
Incorrect punctuation can change the meaning of a sentence. “My brother’s money” belongs to my brother, but “my brothers’ money” belongs to my brothers. A misplaced comma can turn one modifier into two different modifiers. “He arrived for his appointment, late yesterday afternoon,” suggests that he arrived on time for an appointment in the late afternoon. “He arrived for his appointment late, yesterday afternoon,” suggests that he was late for his appointment.
Failure to understand the parts of speech can also cause confusion. If, instead of “I feel bad,” you write, “I feel badly,” it sounds as if you are not very good at feeling.