This one is a little advanced: the grammatical rules of everyday life. Although many would consider rules like the ones illustrated by the video to be redundant, thinking the person we talk to would understand what we mean to say, we have to realize that grammatical mistakes differentiate between true English speakers and those who get by. If your point is to get by in a foreign land, don't bother. If your goal is to communicate as an equal, take the time to learn grammar.