tn The Hebrew text uses the perfect tense of שָׁמַר(shamar) with a vav (ו) consecutive to continue the instruction of the preceding verse. It may be translated “and they shall keep” or “they must/are to keep,” but in this context it refers to their appointed duties. The verb is followed by its cognate accusative—“they are to keep his keeping,” or as it is often translated, “his charge.” This would mean whatever Aaron needed them to do. But the noun is also used for the people in the next phrase, and so “charge” cannot be the meaning here. The verse is explaining that the Levites will have duties to perform to meet the needs of Aaron and the congregation.
tn The form is the Qal infinitive construct from עָבַד (’avad, “to serve, to work”); it is taken here as a verbal noun and means “by (or in) serving” (see R. J. Williams, Hebrew Syntax, 36, §195). This explains the verb “keep [his charge].” Here too the form is followed by a cognate accusative; they will be there to “serve the service” or “work the work.”