Verse 4: So there went up thither of the people about three thousand men: (Lev. 26:17; Deut. 28:25) and they fled before the men of Ai.
[Three thousand] He prudently chooses the greater of the two proposed numbers, evidently judging that the enemy was not to be despised. Now, that men of exceptional fortitude were selected for this expedition, is indicated by the word אִישׁ/man, and by Josephus in his Jewish Antiquities 5. Otherwise the blame would have been assigned unto their idleness, and the affront to God would not be attended to, which was needful (Masius).
[Before the men of Ai, לִפְנֵי] The Septuagint not at all absurdly renders it from the face of the men of Ai, as if this was set down in the place of מִפְנֵי or מִלִּפְנֵי, from before the face of: For it is likely that they did not endure even the sight of the enemy, when, none of them in the fight, but several were smitten in flight (Masius).
They fled before the men of Ai: Not having their usual courage to strike a stroke, which was a plain evidence that God had forsaken them; and a useful instruction, to show them what weak and inconsiderable creatures they were when God left them; and that it was God, not their own valour, that gave the Canaanites and their land into their hands.
Verse 5: And the men of Ai smote of them about thirty and six men: for they chased them from before the gate even unto Shebarim, and smote them in the going down (or, in Morad): wherefore (Josh. 2:9, 11; Lev. 26:36; Ps. 22:14) the hearts of the people melted, and became as water.
[They smote] The sloping and descending way from Ai to Jericho made the fleeing Israelites more liable to injury (Masius). But see, I pray, that all things turn out happily for those that love God, unhappily for those that hate Him. This splendid, as it seems, victory will shortly occasion their destruction; but the Israelites flight will bestow upon them an illustrious victory (Masius).
About thirty and six men; a dear victory to them, whereby Israel was awakened, and reformed, and reconciled to their God and Shield, and they hardened to their own ruin.
[They pursued them from the gate, לִפְנֵ֤י הַשַּׁ֙עַר֙] Toward the faces of the gate (Montanus); before the gate (Jonathan); from the place which was before the gate (Junius and Tremellius); from the gate (Arabic); from before the gate (Masius out of Kimchi). לִפְנֵי/before is in the place of מִלִּפְנֵי, from before; that is, since they had advanced all the way to the gate in order to attack the city, there they were put to flight by the townspeople sallying forth (Masius).
[Unto Shebarim, עַד־הַשְּׁבָרִים] Unto Shebarim (Munster, Pagnine, Tigurinus, Lapide, Bonfrerius, Masius). The place is so called from the breaking of the army of the Israelites; for שָׁבַר signifies to break (Bonfrerius, Masius). To others the name is appellative; and they translate it, unto the breakings (Montanus), until they shattered them (Jonathan, thus the Septuagint in Masius). Perhaps they read, עַד הִשְׁבִּירוּם, until they broke, or crushed, them (Masius). But who would call an army of three thousand crushed because of thirty-six men struck down (Bonfrerius)? Until they were routed (Syriac); unto the place of routing (Arabic). I would rather translate it, unto the pass, as if the name came to the place from the mountain broken there, etc. (Malvenda).
[Fleeing by the descents, וַיַּכּ֖וּם בַּמּוֹרָ֑ד] And they smote them in the descent (Montanus), on the sloping (Junius and Tremellius, Vatablus), understanding, place (Vatablus).
In the going down; by which it seems it was a downhill way to Jericho, which was nearer Jordan.
[It was melted after the likeness of water,וַיְהִ֥י לְמָֽיִם׃ ] And it was unto water (Montanus, Jonathan); that is, as if water (Drusius, Arabic, Syriac). Thus, they shall be unto one flesh, in the place of, as if one flesh (Drusius). To such an extent that it was changed into water (Junius and Tremellius), that is, that it might flow/ melt away like water (Junius). Just like melted ice, which being resolved into water is not able to hold a place (Bonfrerius). Their heart was loosened, trembling, and weak like water (Lapide). They were in an incredibly low frame (Vatablus). As the soul, while it supports itself upon hope, is said to be firm and constant; so, having been cast down from this, it appears soft, fluid, and wavering this way and that. But what is the reason for such perturbation? It is not remarkable that three thousand men were routed by a numerous garrison. But, since they were depending upon the help of God alone, not upon their own strength, they tremble with good reason, since God now appears to stand with their enemies (Masius).
As water, soft and weak, and full of fluctuation and trembling.
 Hebrew: וַיַּעֲל֤וּ מִן־הָעָם֙ שָׁ֔מָּה כִּשְׁלֹ֥שֶׁת אֲלָפִ֖ים אִ֑ישׁ וַיָּנֻ֕סוּ לִפְנֵ֖י אַנְשֵׁ֥י הָעָֽי׃
 אִישׁ/man can convey a sense of fortitude and valor. See 1 Samuel 4:9; 1 Kings 2:2.
 Hebrew: וַיַּכּ֙וּ מֵהֶ֜ם אַנְשֵׁ֣י הָעַ֗י כִּשְׁלֹשִׁ֤ים וְשִׁשָּׁה֙ אִ֔ישׁ וַֽיִּרְדְּפ֞וּם לִפְנֵ֤י הַשַּׁ֙עַר֙ עַד־הַשְּׁבָרִ֔ים וַיַּכּ֖וּם בַּמּוֹרָ֑ד וַיִּמַּ֥ס לְבַב־הָעָ֖ם וַיְהִ֥י לְמָֽיִם׃
 Hebrew: בַּמּוֹרָד.
 A woodenly literalistic reading.
 שְׁבָרִים/Shebarim is related to שֶׁבֶר/breaking/fracture.
 מוֹרָד is related to the verbal root יָרַד, to go down.
 Genesis 2:24: “Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh (וְהָי֖וּ לְבָשָׂ֥ר אֶחָֽד׃).”