This week's Parashah describes the beginning of the process that culminates in the Exodus from Egypt, including Moshe and Aharon's initial appearances before Par’oh.
Chazal teach that when Moshe and Aharon first appeared before Par’oh, they miraculously walked through a heavily guarded doorway into the middle of a large state dinner, and made it to him unharmed. Despite this obvious miracle, Par’oh hardened his heart and ignored them and their talk of Hashem.
The second time that Moshe and Aharon visited Par’oh, they performed another miracle: they turned Aharon's staff into a snake. Par’oh's magicians were able to mimic Aharon's act, however Aharon performed yet another miracle to triumph over Par’oh and his magicians, having his when staff swallow theirs. Nevertheless, Par’oh hardened his heart and again ignored Moshe and Aharon.
The third time that they appeared before Par’oh, they turned water into blood. Again, the magicians did the same, and this time, Moshe and Aharon did not have another miracle to triumph over them.
Rav Yechezkel Levenstein notes that each of these tests was easier for Par’oh to fail than the preceding one. Each of Moshe and Aharon's miracles was smaller than the one before it, so it was increasingly easy for Par’oh to ignore Hashem’s presence. This is Hashem's way of testing a person, and should a person ignore the initial challenge, Hashem leads him down the path which he has chosen, with each step increasing the respective possibility for complete spiritual failure or greatness.
Rav Levenstein z"l cites the Zohar which explains the pasuk "LePetach Chatat Roveitz VeEilecha Teshukato" "...Sin crouches at the door, desiring you..." (BeReshit 4:7) to mean that the Yeitzer HaRa is what is crouching “by the door” waiting to pounce on a person. He goes on to say that at the age of thirteen (or twelve, for girls) is when a person gains the capability to connect with his Yeitzer Tov. These two inclinations join up with the person they reside in. Every person has a choice of which Yeitzer to follow; either he can choose the right path to follow or choose the wrong path to follow.
In addition, Radak states that the Yeitzer HaRa "is at the beginning like a fine string, but at the finish like a thick rope." This means that it is much easier to overcome the Yeitzer HaRa when it first comes than when the Yeitzer HaRa has already had time to settle in. The reason for this is because the Yeitzer HaRa slowly brings us to do evil. One way the Yeitzer HaRa works is that at first, it causes us to do something small, but we still do it. It then gets us accustomed to committing sins until we can't control ourselves anymore. According to Rav Levenstein, Hashem tested Par’oh the same way He tests everyone else, through the Yeitzer HaRa.
We can connect the lesson learned here to a lesson for us nowadays. When we do something bad, even a small thing, it is because Hashem inserted a Yeitzer HaRa into us, and we were unable to control it. Even so, Hashem continuously tests us to see how we deal with our sins, and whether or not we will learn from our past mistakes. Hopefully, we will all realize the dangers of the Yeitzer HaRa, and correct any mistakes that we have made, no matter how small.