Exodus 3:19

I am sure that the king of Egypt will not let you go, no, not by a mighty hand - When the facts detailed in this history have been considered in connection with the assertion as it stands in our Bibles, the most palpable contradiction has appeared. That the king of Egypt did let them go, and that by a mighty hand, the book itself amply declares. We should therefore seek for another meaning of the original word. ולא velo, which generally means and not, has sometimes the meaning of if not, unless, except, etc.; and in Becke's Bible, 1549, it is thus translated: I am sure that the kyng of Egypt wyl not let you go, Except wyth a mighty hand. This import of the negative particle, which is noticed by Noldius, Heb. Part., p. 328, was perfectly understood by the Vulgate, where it is translated nisi, unless; and the Septuagint in their εαν μη, which is of the same import; and so also the Coptic. The meaning therefore is very plain: The king of Egypt, who now profits much by your servitude, will not let you go till he sees my hand stretched out, and he and his nation be smitten with ten plagues. Hence God immediately adds, Exodus 3:20 : I will stretch out my hand, and smite Egypt with all my wonders - and after that, he will let you go.

No, not - See the marginal rendering. Others explain it to mean, Pharaoh will not let the people go even when severely smitten.

3:19 I am sure he will not let you go - God sends his messengers to those whose obstinacy he foresees, that it may appear he would have them turn and live.