你根本不知道我的心痛 怎么翻译成英语。

Definitely you do not know the pain of my heart.

Definitely you do not know how much my heart hurts.

never 是 从来不，表达“根本”要用 definitely。

ache：痛，但是用于病理方面。

pain: 可以表示精神上的痛苦。

you never knew how my heart aches.

you never saw the tears of my heart.

You don't know my heart aches

You don't know my heartache at all.