A grammar mistake occurs when the rules of a language are not followed, leading to incorrect sentence structure or word usage. These errors can include issues like subject-verb agreement, incorrect tense, or misplaced modifiers. For example, saying "She go to the store" instead of "She goes to the store" is a common grammar mistake. Grammar mistakes can affect the clarity and professionalism of writing. They can confuse readers and alter the intended meaning of a sentence. Tools like grammar checkers and style guides can help identify and correct these errors, improving overall communication skills.