It: the pronoun here refers to any first-born animal. In some cases one will have to say “If the first-born animal happens to be unclean, then…” or, as in Good News Translation, the conditional may be transformed to a direct statement.

He shall buy it back: the pronoun he refers to the original owner. This should probably be made explicit in most languages. The verb buy … back has the same meaning as redeemed, so they may be translated the same, if synonyms are not available.

Your valuation: see verse 3.

Add a fifth: see verse 13 and 5.16.

Be sold: the passive formulation may be changed to say “they [indefinite] shall sell it,” or “the priest may sell it to someone else,” or simply “someone else may buy it.”

Quoted with permission from Péter-Contesse, René and Ellington, John. A Handbook on Leviticus. (UBS Helps for Translators). New York: UBS, 1990. For this and other handbooks for translators see here .