Over the last five plagues brought upon Egypt, we repeatedly read that Hashem hardened Pharaoh heart and Pharaoh did not send the nation of Yisrael out of Egypt. Many people ask a very basic question when looking at these parshiyot (Vaera-Bo). It seems like Hashem is forcing Pharaoh to keep the nation of Yisrael in Egypt even though he wants to free them from bondage. Many people wonder how Hashem could take away Pharaoh's free will and force him into keeping the nation of Yisrael as slaves?
It is a great question.
We can follow one school of thought (brought by the Rambam) that Hashem may exercise His very capable ability of repressing one's free will in order to punish them. Hashem wanted to punish Pharaoh for the evil he extended towards the nation of Yisrael, and so Hashem therefore did suppress Pharaoh's will at that time.
There is another interesting way to answer why Hashem hardened Pharaoh's heart. We must remember that Pharaoh was seeing miracles and wonders transpire in Egypt on a weekly basis. He saw water turn to blood, frogs infest the cities, skin diseases thrive in healthy people, wild beasts turn on man, etc. that were all summoned by Moshe on demand. Pharaoh realized that he was dealing with the super-realistic, and so Hashem hardened his heart for him to cope and still make a rational choice. Hashem did not suppress Pharaoh's free will; rather He was enabling Pharaoh to make a free, unfettered, choice. Hashem needed to harden Pharaoh's heart so that Pharaoh could be on the level to truly repent and let the Jews leave Egypt.
Instead he choose the opposite.