When the Daily Dose of Hebrew went over verse 5 about two weeks ago, I learned that the word that my Bible translates as "keeper" is actually a participle from the verb שָׁמַר:
A participle is a verbal adjective, and some of the other translations I follow along in side more with the verbal part and simply translate it as a verb. Latin: "Dominus custodiet te." German: "Der HERR behütet dich." My French translation, on the other hand, renders it as a noun: "Le Seigneur, ton gardien," which is the same as the ESV: "The LORD is your keeper."
I prefer translating this participle as a verb, however, because it more clearly illustrates God's active protection.
