Question:

There is some missing information in the Bible. I read that Moses went down to Egypt with his wife and their two sons (Exodus, chapter 4). Yet later, in chapter 18, I read, “So Moses’ father-in-law, Jethro, took Zipporah, Moses’ wife, after she had been sent away . . . to Moses.” If Zipporah went to Egypt with Moses, when and why did she leave him? What am I missing?

Response:

The only thing that you are missing is a Chumash (Pentateuch) with commentaries. Rashi, the greatest of the Biblical commentators, cites the following Midrashic tradition:

When the Holy One, blessed be He, said to him in Midian, “Go, return to Egypt” (Exod. 4:19), “Moses took his wife and his sons.”

Aaron [Moses’ brother] went forth, met him on the Mount of G‑d, and said to him, “Who are these?” Moses replied, “This is my wife, whom I married in Midian, and these are my sons.”

“And where are you taking them?” Aaron asked. “To Egypt,” he replied.

Aaron retorted, “We are pained by the first ones [who are suffering here], and you come to add to them?”

Moses then said to Zipporah, “Go home to your father.” She took her two sons and went away.

Rabbi Judah Loew (known as the MaHaRaL) explains that it is logical to conclude that it was Aaron who convinced Moses to send his wife and children back. We can safely assume that Moses did not decide to send them back once he had arrived in Egypt, as that would have sent a very unreassuring signal to the suffering people: at the very same time that Moses promises salvation, he sends his own family to a safer place. It also could not be that Moses decided to send his family back on his own volition before his arrival in Egypt, as he himself had decided to bring them. The only option we are left with is that Aaron—who we know had gone to meet Moses in advance of his arrival—dissuaded him from bringing his family along.