sh-m-r — to guard, to keep, to observe
ש.מ.ר (shamar) covers guarding a place, keeping a commandment and preserving something over time — one verb where English needs three.
The word family
Every word below is built on ש.מ.ר. Same three letters, different vowels and prefixes.
שׁוֹמֵר שַׁבָּת (shomer shabbat) — someone who keeps the Sabbath, using the same participle as a night watchman.
A hospital shift and a Temple watch are the same word.
In the text
לְעָבְדָהּ וּלְשָׁמְרָהּ
l'ovdah ul'shomrah
To work it and to keep it.
יְבָרֶכְךָ יְהוָה וְיִשְׁמְרֶךָ
Yevarekhekha Adonai v'yishmerekha
May the LORD bless you and keep you.
What I tell students about this root
English forces a split here that Hebrew never made. "Guard the gate" and "keep the commandments" are one verb, so keeping a law reads as standing watch over it rather than obeying it. When you see shamar in a legal passage, the guarding sense has not gone anywhere.