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grammar question

You would probably just say "I still don't know how to do it."

I didn't know how to do it before, and I still don't know how (to do it).

It is more natural sounding to leave the part in parentheses (to do it) off the sentence, although both are correct.

Hello Wei Jia Chen. How are you?

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how do i say sentence express that : i did't know how to do it before and I don't know either for now and i keep don't know it.

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Your sentences are already very close to saying exactly what you intend to say.
I would write them this way:

"I didn't understand it before."

"I do not understand it now."

"I probably will not understand it in the future."

or...

"I did not know how to do it before.
I do not know how to do it now.
I probably will not know how to do it in the future."

--Warm Regards, Bruce

I have never, ever understood it. And I still don't understand it today.