There is a similar question is asked here
But here I am asking other way around. Can I use magento 2.0 module at magento 1.9?
Magento 2 architecture completely different from Magento 1, so we can't simply copy and paste the extension.
app/code/local or community pool to carry all the custom extensions.
but in M2
will carry our custom extensions.
There is an file structural changes as well like namespaces and module structure also differs from each other. Take a look at this Magento 2 Extension Development devdocs