Explaining all the ins and outs of modal theory in one post might be a bit much to ask. It's a big topic.

Do a search for modal thoery online. There's probably some good info already there and it would save people here the time of retyping existing info just for this thread.

Tell you what, if you can't find anything explaining modal theory already online, give me your life's story in 2000 words and I'll give you modes in a nutshell. ;)