conditional if or unless

[valtango]

Here we go again, why is it so difficult to explain English to anyone else? I am just teaching my "students" the conditional, and the correct use of if or unless. What is the difference? I have looked at so many web sites,and my little pocket grammar book, and even the information is cintradictive. I found a really simple basic rule, if goes with negative and unless with positive. eg. If you don't wear your coat you will be cold. Unless you wear your coat you will be cold. This applies perfectly to the rule. Then.. If we heat water enough, it begins to boil. So here the rule has gone..This is if in a positive sentence. I really need an easily understandable rule to give to my students, who are mostly senior citizens trying to get a basic understanding of English. Can anyone help me please? Can you maybe also reccomend to me a really good and very informative grammar book? I am using oxford pocket english grammar, which is mostly very good, but sometimes I just need more information. Thank you.
Valerie

[Tdol]

Unless can replace if + a negative verb, but that doesn't mean that if cannot be followed by a positive verb.
A good grammar- Michael Swan Practical English Usage:
http://www.usingenglish.com/amazon/us/0194420981.html
It covers both standard grammar points and those tricky little questions people ask.