Here is the example, which is right?
It's important he know this.
It's important he knows this.
It's important he has known this.
or, those above all right, but express different meaning.
The third form of the sentence could be grammatically correct but expresses an unlikely thought, implying he no longer knows this, but it is currently important that he knew it some time in the past. It seems to be motivating the listener to take some action to ensure that the person who currently may not know it, did know it in the past. This is impossible without time travel.
The final "s" in "knows" is third person not plural, mainly because plurality applies to nouns not verbs I suppose.
As for the subjunctive: I can't do better than point you at Wikipedia's article
Grammar Monster suggests "Through common usage, the non-subjunctive forms of verbs are gradually replacing the subjunctive forms."