你根本不明白翻译成最正确的英文,不要用翻译工具

You don't understand!

语气强烈些就有“根本”的意思了，不用特意加上别的词。
如果非要加，可以说
You don't understand at all.
You don't understand a thing.

You never know why.

You don't even understand.

You don't understand completely

you know nothing.
you don't know any thing.

You don't understand at all.