Don't know if this helps, but it helps me. For when to use which and that - if the word follows a comma then you use which, if there isn't, use that. eg The cat, which was fat, couldn't fit through the flap. The cat that was fat couldn't fit through the flap. My big problem is with its and it's. I just write up the document and when finished, do a Control + F, and do a find for each its and then for the it's, checking to see if it's used possessively (its) or not (it is = it's).