"I need to learn hard." is not a good sentence. "I need to practice a lot." would work for this situation since playing violin is an activity. In another situation you might say "I need to study hard." ex; "I have an important test next week, I need to study hard."

That sentence doesn't make much sense. I'm not exactly sure what you mean, could you explain a bit more? Maybe you mean to say "I need to study hard." ?

Thank you, Bob!!

Marco - probably not in this situation as playing the violin is a practical skill. You could say "I have an exam tomorrow so I need to hit the books." Helen - I would use the verb 'study' in your sentence or possibly 'practice' - With practice I would use the adverbial phrase 'a lot' afterwards - "I need to practice a lot."

No, when we need to learn, then we must study hard. In the case of a violin, you need to practice hard or practice religiously or diligently. "Learn hard" has no meaning in English. We don't say this.