en_tn/deu/23/09.md

When you march out as an army against your enemies, then you must keep yourselves

Moses speaks here to the Israelites as if they were one man, so the words "you" and "your" are singular. (See: rc://en/ta/man/translate/figs-you)

against your enemies

"to fight against your enemies"

keep yourselves from every evil thing

"keep yourselves away from all bad things"

any man who is unclean because of what happened to him at night

This is a polite way of saying that he had an emission of semen. AT: "any man who is unclean because he had an emission of semen while he was asleep" (See: rc://en/ta/man/translate/figs-euphemism)