I've been perplexed by this for awhile but I haven't really been filled with enough enthusiasm to unravel this question: What is the difference between A and An? I know that it sounds right in various situations to say either one, but I have no idea why you say either, that is, grammatically speaking. I have a tomato. I had a tomato. I will have a tomato. Okay. So it doesn't have to do with tense. An approach<-I've had a brain cramp so this is the only sentence with an an that I could think of. Arghghh! Damnit. I'm stuck.