What are common English language grammar rules?

The correct use of an apostrophe is something that people often misuse. Just use the following rules. Use an apostrophe; 1) to denote possession, e.g. Henry's ball (unless using a third person pronoun e.g. "his" or "her", which would be "his ball" or "her ball"; or 2) for a contraction, e.g. it is / it's, do not / don't, cannot / can't.