That it may be known when a thing is clean or unclean.

to teach when there is uncleanness, and when it is purified: this is the law of leprosy.

to direct in the day of being unclean, and in the day of being clean; this is the law of the leprosy.'

To make clear when it is unclean and when it is clean: this is the law about the disease of the leper.

so that it can be known at what time a thing is clean or unclean.

Ad docendum quid agendum, quo die immundus et quo die mundus declarandus est.

To teach when it is unclean, and when it is clean,.... A man, his garment, or his house; for it respects them all, as Aben Ezra observes; which was the business of the priests to teach men, and they by the above laws and rules were instructed how to judge of cases, and by which they were capable of pronouncing persons or things clean or unclean:
this is the law of leprosy; respecting every sort of it, and which is very remarkably enlarged upon.

To teach - To direct the priest when to pronounce a person or house clean or unclean. So it was not left to the priests power or will, but they were tied to plain rules, such as the people might discern no less than the priest.