Simple question as title, Can i still use magento 1.9 module/theme at magento 2.0? Is magento 2.0 a completely new system and not compatible with 1.9?
It is not possible to use Magento 1.x extensions and design themes with Magento 2 without code refactoring.
There is the official code migration tool which can help the developers to migrate theirs extensions and design themes: https://github.com/magento/code-migration
But the tools is for experienced developers only.