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."