Numbers 4:18

4:18 “Do not allow the tribe of the families of the Kohathites to be cut off ¹  from among the Levites;

Numbers 8:6

8:6 “Take the Levites from among the Israelites and purify ²  them.

Numbers 8:18

8:18 So I have taken the Levites instead of all the firstborn sons among the Israelites.

[4:18]  ¹ sn The verb is simply the Qal, “do not cut off.” The context calls for a permissive nuance – “do not let them be cut off.” It was a difficult task to be handling the holy things correctly; Moses and Aaron were to see to it that they did it right and did not handle the objects, that is, Moses and Aaron were to safeguard their lives by making certain that proper procedures were followed.

[8:6]  ² tn The verb טָהַר (tahar) means that Moses was “to purify” or “to make ceremonially clean” the Levites so that they could enter the sanctuary and do the work prescribed for them. Whatever is “unclean” is not permitted in the sanctuary at all.