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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.

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up vote 1 down vote accepted

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."

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thank you! I see. Is the first sentence subjunctive mood? so, the first sentence use the basic form in verb. – lovespring Jan 19 '11 at 9:34
I think you are right. – RedGrittyBrick Jan 19 '11 at 9:42
Agreed. Here's another link to the present subjunctive: bbc.co.uk/worldservice/learningenglish/grammar/learnit/… – Tragicomic Jan 19 '11 at 10:20
@Tragicomic: I have revised my answer. I feel uncomfortable with the first sentence although I believe it is a grammatically correct use of the subjunctive. This may reflect either that use of the subjunctive is in decline in my part of the Anglosphere or possibly that my use of English is not strictly correct. – RedGrittyBrick Jan 19 '11 at 10:33

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