Exodus 13:10

13:10 So you must keep ¹  this ordinance at its appointed time from year to year. ² 

Numbers 18:15

18:15 The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, ³  and the firstborn males of unclean animals you must redeem.

[13:10]  ¹ tn The form is a perfect tense with the vav (ו) consecutive, functioning as the equivalent of an imperfect of instruction or injunction.

[13:10]  ² tn Or “every year,” or “year after year.”

[18:15]  ³ tn The construction uses the infinitive absolute and the imperfect tense of the verb “to redeem” in order to stress the point – they were to be redeemed. N. H. Snaith suggests that the verb means to get by payment what was not originally yours, whereas the other root גָאַל (ga’al) means to get back what was originally yours (Leviticus and Numbers [NCB], 268).