70 The sequence of the instruction from God uses the perfect tense with vav (ו), following the preceding imperfects.
71 The instantaneous use of the perfect tense fits well with the prophetic announcement of what Yahweh said or says. It shows that the words given to the prophet are still binding.
72 The metaphor uses the word “son” in its connotation of a political dependent, as it was used in ancient documents to describe what was intended to be a loyal relationship with well-known privileges and responsibilities, like that between a good father and son. The word can mean a literal son, a descendant, a chosen king (and so, the Messiah), a disciple (in Proverbs), and here, a nation subject to God. If the people of Israel were God’s “son,” then they should serve him and not Pharaoh. Malachi reminds people that the Law said “a son honors his father,” and so God asked, “If I am a father, where is my honor?” (Mal 1:6).

In other versions

Then you shall say to Pharaoh, ‘Thus says the Lord, Israel is my firstborn son,

Then say to Pharaoh, ‘This is what the Lord says: Israel is my firstborn son,

And you will say to Pharaoh: This is what the Lord says: Israel is my firstborn son.

Then you will tell him, ‘This is what the Lord says: Israel is my firstborn son.

And you must say to Pharaoh, ‘Thus says Yahweh, “Israel is my son, my firstborn.”

And thou shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn:

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