Not sure when to use that and when to use which? Have no idea what I’m saying when I admonish you for using all of that passive voice?? Wish you knew when to lie and when to lay??? A great tool for these and everything else grammar-related is Grammar Girl. Read it online or (my favorite) download it as a four-minute weekly podcast.Not everything GG says is AP-Style correct, but it is sooo darn close and still very much worth your time.