You shall observe means to watch or guard (see verse 17). The You is plural. This rite uses the word davar, meaning “word” or “event.” So Good News Translation has “You must obey these rules.” (Durham has “this command,” and New International Version “these instructions.”) As an ordinance occurs in verse 14 and means “a statute” (Revised English Bible) or “an institution” (New Jewish Publication Society’s Tanakh). For you and your sons (surprisingly, the you is singular) refers to all the Israelites and their “children.” Grammatically for ever goes with observe rather than ordinance (but see New Revised Standard Version, “a perpetual ordinance”) and extends the meaning of your sons to “your descendants” (New American Bible). It is also possible to express this verse as “You and your children must continue to celebrate this Passover festival each year forever.”

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .