use of 'that' and comma

Azah said:

a. How foolish you were that is my pain.
b. How foolish you were, is my pain.

I think both are incorrect. There are no need of that and comma. And I can use '!' after these two sentences.

They are both incorrect. Are you asking us whether you can use an exclamation mark or telling us that you can?

c. You don't know that how important the matter will/would be for me.
You don't know how important the matter is for me.

d. How the matter will/would be for me you don't know that.
I have no idea what you're trying to say with this but it is certainly not a grammatical sentence.

If use of 'that' is redundent for a and b, is it correct to use 'that' in last two examples?

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