Any sort of sentence can end in an explanatory の or のだ expression.
Tae Kim's Guide to Japanese Grammar has a section, but before I refer you there, I want to warn you that his English translation examples are often so literal that they aren't correct English at all. That can be difficult or confusing if English is not your first language.
Anyway,
の as explanation:
http://www.guidetojapanese.org/particles3.html#part5
Tae Kim's Grammar Guide in French:
http://www.guidetojapanese.org/french/index.html
jgram.org is also a useful site in that they have an entry for almost every grammar point, but unfortunately, those entries are often terse or confusing, but many people find the place useful anyway if only because it least it says -something- about anything you could think of (if you phrase your search carefully):
http://jgram.org/pages/viewOne.php?tagE=no+da
I'd also recommend A Dictionary of Basic Japanese Grammar. This forum's sponsor site sells it, although it can of course, also be found at any major bookseller. There's also an intermediate and advanced volume when you feel ready for them:
http://www.thejapanshop.com/A-Dictionar ... =salesrank
Also, of course, I use a combination of dictionaries,
The firefox plugin rikai-chan which pops up the pronunciation of kanji and the EDICT definition on mouseover:
http://www.polarcloud.com/rikaichan/
The EDICT J-E database has its home with WWWJDIC
http://www.csse.monash.edu.au/~jwb/cgi- ... dic.cgi?1C
Unfortunately, while it's a wonderful resource, EDICT is really an -enormous- glossary and not a proper dictionary with examples and structured definitions, so I often visit yahoo for their 和英 (japanese-english) and 国語 (national language→Japanese) dictionaries:
http://dic.yahoo.co.jp/
(I also use a handheld 電子辞書 but mine is quite old and I don't have any recommendations for one in particular. A lot of people are preferring to buy dictionary applications on smartphones these days.)