True or False: Grammatical errors are endemic to the web. The statement is false. To say grammatical errors are endemic to the web is to say that grammatical errors don't occur elsewhere. Endemic is a word that contains limitations within itself; it is an adjective of boundaries.
When it comes to grammar, there seem to be no boundaries on the number of possible errors. Some are typos. Most are errors.
How do you know what's right or wrong? You consult a style manual. Which one? The Chicago Manual of Style, 15th edition, is accepted in nonfiction circles. The MLA Handbook, sixth edition, is the one I was accustomed to during my days as an English major. Pick one style sheet, and stick with it. The key to success with either one is taking the time to pull the book down from the shelf and confirm whether your usage is right or wrong. Think of it as an investment.