שָׁמֹר Deut. 27:1

[Isaac Fried]

We read there
וַיְצַו מֹשֶׁה וְזִקְנֵי יִשְׂרָאֵל אֶת הָעָם לֵאמֹר שָׁמֹר אֶת כָּל הַמִּצְוָה אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם הַיּוֹם
KJV: "And Moses with the elders of Israel commanded the people, saying, Keep all the commandments which I command you this day"
It appears that שָׁמֹר is in what English grammar calls a qal infinitive form, a form said to be the basic form of a verb, without an inflection binding it to a particular subject or tense, or in Hebrew: צורת פועל שאין לה זמן וגוף היא מביעה מהות מופשטת של הפעולה
Not so, thinks I. The form is
שָׁמֹר = שמ-הוּא-ר including the internal personal pronoun הוּא for the person(s) urged to perform the act.

Isaac Fried, Boston University