This document provides 15 examples of common grammar mistakes and the correct rules to apply. It addresses mistakes involving verb tense when using "if" conditionals, subject-verb agreement with quantifiers like "every" and "each", proper use of conjunctions like "although" and "but", subject-verb agreement with collective nouns like "police", and differences between similar words like "your" vs. "you're", "amount" vs. "number", "then" vs. "than", and "beside" vs. "besides". For each case, it gives the incorrect and correct versions with a brief explanation of the applicable grammar rule.