What is the definition of a Mofeis?
Refer to 7:3:3:1-5.
What is the definition of "Tanin"?
Rashi: "Tanin" is a snake (or a viper
Whose staff did Hashem instruct Aharon to take?
Why did the staff turn specifically into a snake?
Targum Yonasan: It was a prediction that the whole world were destined to hear the cries of the Egyptians when Hashem would break them, just as the whole world heard the screams of the snake (in Gan Eden) when Hashem cut off its hands and legs.
Da'as Zekeinim: Because Par'oh was vain, and therefore became known as 'the great Tanin'; and because, just as a snake twists and turns as it moves along, so too, did Par'oh twist and turn. 1
Rosh (in 6:3): Par'oh is called a snake - "ha'Tanin ha'Gadol." 'You made yourself like a snake, and swallowed twelve Matos (tribes) - you will eventually expel them, and I will return you to Tohu va'Vohu (nothingness).
Oznayim la'Torah: It was Moshe's staff that turned into a crocodile (a sea-serpent), which is the translation of 'Tanin'