When did God start hardening Pharaoh's heart?
God forecasted the hardening before plague one (Exodus 4:21 and 7:3), but the first verse in the narrative where YHWH is the explicit grammatical agent of the hardening is Exodus 9:12, the sixth plague — before that, the verbs are either intransitive or show Pharaoh as his own agent.
This is one of those questions where the popular summary is close but not quite right — and the gap matters.
Most people say something like "God took over the hardening partway through" and put the dividing line at plague six. That is roughly correct. But the full answer has two parts, and you need both.
Part one: the forecasts. Before plague one ever lands, God announces the hardening twice. At the burning bush he says "I will make his heart firm" (Exodus 4:21) — the Hebrew verb is chazaq (חָזַק, H2388) in the Piel, which is an agent-explicit causative: God as the stated subject, Pharaoh's heart as the object. Then again in the commissioning speech of Exodus 7, the very chapter where the plagues begin: "I will harden Pharaoh's heart" (Exodus 7:3), this time qashah (קָשָׁה, H7185) in the Hiphil. Two announcements, God in the first person, before Pharaoh has refused even once. The hardening is declared as strategy, not improvised as a reaction.
Part two: the narrative. When the plagues actually begin, through the first five of them, God is not named as the agent of any hardening in the narrative. What the Hebrew shows instead is either an intransitive verb — "Pharaoh's heart became firm" (7:13, 7:22, 8:19, 9:7), no agent stated — or Pharaoh named explicitly as the one who "made his heart heavy" (8:15, 8:32). These early hardenings are not described as God doing something to Pharaoh; the heart simply stiffens, or Pharaoh himself stiffens it.
The change comes at plague six, the boils. Exodus 9:12 reads: "YHWH made firm the heart of Pharaoh." This is the Piel of chazaq with YHWH as the grammatical subject — the first time in the plague narrative that God is the named agent of the hardening. Dead Sea Scroll fragments and the ancient Greek both carry the same reading. From that point forward, through plagues eight (locusts), nine (darkness), ten (firstborn), and into the sea, YHWH is the consistent explicit subject (Exodus 10:20, 10:27, 11:10, 14:8). The Piel active participle at 14:17 — "I am hardening the heart of Egypt" — is the most intensive form of all, God as the ongoing present agent as Egypt enters the sea.
So: the forecasts are as early as it gets (before plague one), and the first narrative hardening where God is the explicit agent is plague six (Exodus 9:12). Those are two different questions, and both answers are correct. God announced it from the start, and the narrative shows Pharaoh hardening himself through five plagues before YHWH's explicit hardening begins in plague six — while both remain inside the frame God set before the contest opened.
Did God harden Pharaoh's heart, or did Pharaoh harden his own?
Both — and the Hebrew grammar shows each one precisely, with God forecasting the hardening before it begins and Pharaoh acting as his own agent through the first five plagues before God becomes the explicit subject at plague six.
What does Romans 9 mean by 'he hardens whom he wills'?
Paul is drawing a direct theological principle from the Exodus plague cycle, reading the Greek of Exodus 9:12 and 9:16 together: the same God who made Pharaoh's heart firm for the display of his name makes the same move in every age, and Paul says so using the same Greek verb the Septuagint used for the hardening of Pharaoh.
Why did Aaron's staff turn into a serpent, and how did the magicians copy it?
The sign is a direct confrontation with Pharaoh-as-dragon on his own ground: the word used in Exodus 7 is not the ordinary word for snake but tannin, the Hebrew chaos-creature, and Ezekiel later identifies Pharaoh himself as 'the great tannin lying in the midst of his Nile streams' — Aaron's staff-dragon swallowing the magicians' staff-dragons is the whole contest in miniature.
Why did God turn the Nile into blood?
The plague targets Egypt's source of life and divine status — the Nile was deified in Egyptian religion — and the stated purpose in Exodus 7 is not punishment for its own sake but forcing Egypt to know who YHWH is, answering Pharaoh's own boast that he does not know YHWH.