My friend had an argument with his Girlfriend about one statement. He asked me for the elaboration of its being either correct or wrong. What I simply had for my reply was; I wasn't certain about my answer. So, I please here come for that which I hope will gladly be answred soon.

The statement; Forget about that matter and lets talk about matter that matters here.

She told him that when he was speaking about her cordial relation with one of his friend and which he didn't understand or the former was dating her.
Matter = issue, idea

Matter = is important

So: Forget about that matter (issue), and let's talk about matters (issues) that matter (are important) here.

The second matters should be plural, so the verb matter shouuld take the plural form.
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Thanks for that. Yet I'm a bit unclear because; as you said in your reply that second matter should be plural. Why should it be? Or you mean the third one as the verb of the sentence.

Again, how verb can take plurality?

I think the version given by GG is the one that is idiomatic; that is how the sentence is phrased when spoken by people in her native country.

GG's: Forget about that matter and talk about matters that matter here.

Your possible version: Forget about that matter and talk about the mattr that matters here.

IMO, your possibly intended?? sentence assumes that you know which (specific) issue is important and you want to talk about it specifically but in the real world, one doesn't know what issues are important and how many issues are to be discussed next and that is why the subject is in plural and the verb like that accordingly in the underlined clause.
Yes, Believer, exactly.

Let's talk about matters (plural) that are important.


Let's talk about the matter (singular) that is important (to us right now).

The verb matter would (like all verbs) have a singular and plural form: They matter (plural), it matters (singular).

I would advise against using matters (issues) and matter (is important) in the same sentence in general speech.
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Uncountable thanks to Believer and GG.

We learners of English, without people like you...uh..I don't know how we're going to learn this vast language. Keep the ball rolling.