What is your preferred grammar source / guide?
I am trying to understand the grammar of the following sentence 「通らなくてはならない道。」

EDIT: Fixed a typo on my sentence. The sentence is the title of the third episode from a J-Drama called Bambino.

http://www.tatoeba.org

Thanks, I was looking for something more like a grammar resource with explanations and examples for specifics points, in this case something like らなくて maybe, I have no idea how to break the sentence.

It looks like an error.
らんくて is the て form negative ending of a verb, but I'm not aware of 道 forming such a verb.
Maybe it's an error for 満ちる which ought properly be 満ちなくて or something, can't tell without any context.

(Oh, the answer below mine is a -much- better guess, I like it.)

It couldn't be 通らなくてならない道, could it? I can only think it must be a typo of some kind...

I tried googling it and nowt came up...
Tae Kim has a good grammar database but I prefer the dictionaries of Japanese grammar series by Makino and Tsutsui

Community based, but check out jgram.org

The second hit on google for なくてはならない contains the following:

なくてはならない 無くてはならない

《人が主語》 cannot do without something
《物が主語》 be indispensable 《to》 be necessary 《to, for》.

http://www.jgram.org is one of the best japanese grammar sites out there.
The grammar points are organized by JLPT level.

Dictionaries of Basic/Intermediate/Advanced Grammar

However, none of these resources will help you parse things you don't know. That's one of the reasons it's so hard to skip a structured introduction to the grammar. Unless someone explains it to you or you see it beforehand in a book, you're never going to be able to figure out that 通らなくてはならない道 is 通る in negative -te form plus ならない, which is a special grammar point, modifying 道.