The usage of helpful in and helpful to.

[jessephine]

I've been told that(by my classmate) It's helpful to improve their skills is wrong,so is it correct to replace this sentence by It's helpful in improving their skills?
When and where to use these two phases?And how to differentiate them?
Please give some examples to explain it.

[abaka]

It's all a question of idiom.

"Is helpful to doing something" is not good idiom. "Helpful in doing something" is a little better; better yet is "helpful for"; but best of all, why not just say "helps"?

X "This is helpful to running." Poor.

V "This is helpful to him" (a person doing something, not the abstract action). Good.

-- "This is helpful in running". Passable, but a little odd.

V "This is helpful for running". Acceptable.

V "This helps him". Good English!

V "This helps him run". Good!

V "This helps his running". Good!

X "This helps running". Not so great.

V "This helps in running."

V "This helps to run". Good!

I'm not going to attempt an explanation of why some of these phrases sound better than others, but they just do.

[bhaisahab]

I've been told(by my classmate) that It's helpful to improve their skills is wrong,so is it correct to replace this sentence by It's helpful in improving their skills?
When and where to use these two phases?And how to differentiate them?
Please give some examples to explain it.

"It's helpful to improve their skills" is a correct sentence. It means that the act of improving their skills is helpful.
It would be helpful to improve the skills of a workforce if you want to raise the quality of their output, for example.
"It's helpful in improving their skills" is also a correct sentence. It refers to something, a method, that is helpful towards improving their skills.
If we take the workforce in my first sentence as an example, the method that you use to improve their skills and thus raise the quality of their output is helpful.