Speaketh constantly - His testimony abides evermore who repeats simply what he has heard, whether from the lips of men or from the voice within, in contrast with "the false witness."

A false witness shall perish: but the man that heareth (m) speaketh constantly.
(m) He may boldly testify the truth that he has heard.

A false witness shall perish,.... As witness he shall perish in his reputation, no credit shall be given him, he shall not be admitted an evidence, or a witness in any cause, being found a false one; and as a man, he shall be punished in body or estate by the civil magistrate, and his soul shall perish eternally, unless he has true repentance for his sin: or, a witness of lies shall perish (k) it may be applied to any teacher of false doctrine; and to the man of sin, and his followers, that speak lies in hypocrisy; everyone that loves and makes a lie shall die the second death, and be excluded from eternal happiness, Revelation 21:8;
but the man that heareth; before he speaks, and speaks what he hears, and does not devise things himself; but witnesses the truth, and nothing else, to the best of his knowledge:
speaketh constantly; invariably and consistently, what is all of a piece, and by which he ah, des; or "continually", as Jarchi; or "for ever"; he is made use of as a witness as long as he lives, whenever there is occasion for him; the Vulgate Latin version renders it, "he speaks victory"; his testimony, being true and valid, carries the cause: it, nay be applied to a faithful teacher, who hearkens to the word of God, and speaks according to that; such an one speaks out, he doctrine of the word constantly, boldly, with certainty, without any hesitation or staggering.
(k) "testis mendaciorum", Montanus, Junius & Tremellius, Cocceius, Gejerus, Michaelis, Schultens.

The doom of a false witness is certain.

(Compare Proverbs 19:5).
that heareth--or heeds instruction, and so grows wise.
speaketh constantly--or sincerely (compare Habakkuk 1:5), and hence is believed (Proverbs 12:19; James 1:19).

28 A false witness shall perish;
But he who heareth shall always speak truth.
The lxx translate 28b by ἀνὴρ δὲ φυλασσόμενος λαλήσει. Cappellus supposes that they read לנצר for לנצה, which, however, cannot mean "taking care." Hitzig further imagines שׂמח for שׁמע, and brings out the meaning: "the man that rejoiceth to deliver shall speak." But where in all the world does נצר mean "to deliver"? It means, "to guard, preserve;" and to reach the meaning of "to deliver," a clause must be added with מן, as מרע. When one who speaks lies (עד כּזבים), and a man who hears (אישׁ שּׁומע, plene, and with the orthophonic Dagesh), are contrasted, the former is one who fancifully or malevolently falsifies the fact, and the latter is one who before he speaks hears in order that he may say nothing that he has not surely heard. As לב שׁמע, 1-Kings 3:9, means an obedient heart, so here אישׁ שּׁומע means a man who attentively hears, carefully proves. Such an one will speak לנצח, i.e., not: according to the truth, and not: for victory (Aquila, Symmachus, Theodotion, εἰς νῖκος), i.e., so that accomplishes it (Oetinger); for the Hebrews. נצח has neither that Arab. nor this Aram. signification; but, with the transference of the root meaning of radiating or streaming over, to time, continuous existence (vid., Orelli, Synonyma der Zeit und Ewigkeit, pp. 96-97), thus: he will speak for continuance, i.e., either: without ever requiring to be silent, or, which we prefer: so that what he says stands; on the contrary, he who testifies mere fictions, i.e., avers that they are truth, is destroyed (28a = Proverbs 19:9, cf. 5): he himself comes to nothing, since his testimonies are referred to their groundlessness and falsity; for שׁקר אין לו רגלים, the lie has no feet on which it can stand, it comes to nothing sooner or later.

Heareth - He witnesses nothing but what he has heard or seen. Constantly - Always affirms the same thing.