I find that aspect and tense are hard to pin down in my conlangs. Is there a good tutorial on tense and aspect on the web anywhere? Come to think of it, what books would people recommend? I know of the ones by Comrie. What I'd like is something to explain how different languages handle tenses and aspects -- what sorts of distinctions they make and what things they collapse together. I'd also like to read about the interaction between tense and aspect, and perhaps voice and mood.