I just don't get how some grammar is considered correct, while in other circumstances it's a run-on sentence. I also don't understand how sometimes a semicolon is necessary while other times, it's perfectly okay to use a comma. For example, "If you examine a document you have written recently, you are likely to find many such sentences; they’re so common that you don’t even realize you are writing them." Why is there a comma after 'recently,' if there are no conjunctions to connect the sentences. Wouldn't that be considered a run-on sentence? Also why is there a semicolon after the word 'sentences' and not a comma or period? I do not understand.