B-Hebrew: The Biblical Hebrew Forum

Mira,
Indeed, שָמוֹר is imperative צִווּי as is שְמוֹר, yet there seems to be a slight difference in their biblical usage, but not consistent. It seems to me that a direct command to a specific person is שְמוֹר as in Prov. 7:1
בְּנִי שְׁמֹר אֲמָרָי וּמִצְותַי תִּצְפֹּן אִתָּךְ
KJV: "My son, keep my words, and lay up my commandments with thee"
A command, or exhortation, to a group comes as שָמוֹר as in Deut. 16:1
שָׁמוֹר אֶת חֹדֶשׁ הָאָבִיב וְעָשִׂיתָ פֶּסַח לַיהוה אֱלֹהֶיךָ
KJV: "Observe the month of Abib, and keep the passover unto the Lord thy God"
The reason for the different forms in the HB is that at the time the niqud was introduced there were different reading traditions to be accommodated, and also the deeper reason of Hebrew having been developed differently over time and place.

Isaac Fried, Boston University

Just because it touches on what's being discussed here.

Gesenius & Kautsch wrote:[§133bb]
The infinitive absolute is most frequently used in this way, corresponding to the infinitive of command in Greek, &c.:—

(α) For an emphatic imperative, e.g. שָׁמוֹר (thou shalt, ye shall), observe Dt 5:12; זָכוֹר (thou shalt) remember, Ex 13:3, 20:8 (the full form occurs in Dt 6:17 שָׁמוֹר תִּשְׁמְרוּן;‎ 7:18 זָכֹר תִּזְכֹּר);‎ Lv 2:6, Nu 4:2, 25:17, Dt 1:16, 2 K 5:10, Is 38:5, Jer 2:2, followed by a perfect consecutive; Jos 1:13, 2 K 3:16, Is 7:4, 14:31 (parallel with an imperative; in Na 2:2 three imperatives follow). But הַבֵּיט ψ 142:5 may be only an incorrect spelling of הַבֵּט imperative.