natural?

It's OK, but it's not a complete thought.

Why isn't it a complete thought? It's not how I'd phrase it, but I can't find anything wrong with it.

You won't be able to do X only though Y.

You won't be able to learn Latin by sleeping with the book under your pillow. Same construction, isn't it? (Of course, I may be missing something. I was up late last night.)

Grammar GeekWhy isn't it a complete thought? It's not how I'd phrase it, but I can't find anything wrong with it.

The phrase "only by" begs a clause.... gramatically, it was OK, but I was left hanging for a completion
You won't be able to learn English successfully only by inspiration or studying hard before the exams; you will have to p

Ah!

Yes, I agree. Semantically incomplete, but grammatically okay.

So at least it's grammatically OK. Good.
Thank you both! 

Hi,
A small extra comment.

It seems strange to me to speak of learning English by inspiration. How one would do that is not clear to me.

Clive

CliveIt seems strange to me to speak of learning English by inspiration. How one would do that is not clear to me.

I took this to be the point, that you have to study, you can't just wait around for some magical inspiration to bestow mastery of the language on you.

TakaYou won't be able to learn English successfully only by inspiration or studying hard before the exams.

The only by bothers me. And so does the lack of parallelism between inspriration and studying, and between the lengths of the phrases. Doing the least damage possible to the original, I would write
You won't be able t