en_tn_lite_do_not_use/num/09/06.md

became unclean by the body of a dead man

This implies that they touched the dead man, which made them unclean. You can make clear the full meaning of this statement. Alternate translation: "became unclean because they touched the body of a dead man" (See: rc://en/ta/man/jit/figs-explicit)

unclean

A person who God considers spiritually unacceptable or defiled is spoken of as if the person were physically unclean. (See: rc://en/ta/man/jit/figs-metaphor)

keep the Passover

Here the word "keep" means to observe. Alternate translation: "observe the Passover" or "celebrate the Passover" (See: rc://en/ta/man/jit/figs-idiom)