Three reasons are important why young people need to practice sports.

[Gaby2003]

Why is the following sentence grammatically incorrect?

Three reasons are important why young people need to practice sports.

 

[Tarheel]

"Three reasons are important" is grammatically correct. (But I don't know what it means.) However, "why young people need to practice sports has no connection to the rest of the sentence.

 

[Gaby2003]

Thanks! The teacher told me I had to write "There are three reasons why young people need to practice sports" because what I had written was wrong. But I didn't understand why

 

[probus]

What you wrote is not gramatically wrong (as @Tarheel already said) but it is unnatural, which means it is not what a native speaker would choose. That's why your teacher wants you to write it in different words.