My phone still under warranty and my screen cracked, Can I change it for free?

No. Accidental damage is not covered under warranty, as its not a manufacturing defect.

You can get the screen fixed for the out of warranty repair price.

iPhone Screen Repair & Replacement - Official Apple Support

Depends on how it got that way. Only in rare cases would it be covered under warranty, and if it was a result of it being dropped that's not going to happen.

No.

Accidental damage is not covered on a standard warranty.