פּירוש אויף שמות 7:10

לתנין means A SERPENT.

And it became a tanin In the wilderness, which is a place of snakes, the staff was turned into a snake; but in Mitzrayim, which is a place of taninim, it was turned into a tanin. A tanin [תנין] is a "crocodile" in this language, like in Yechezkel 29:3 "The great crocodile [hatanim התנים] that crouches in the midst of his streams". And he didn't do the second sign in front of Par'oh, to bring his hand out from his breast and behold it was afflicted with tzara'at like snow, because if he had been afflicted with tzara'at they would have removed him from Par'oh's presence, and something like this is in Bereishit 50, that Yosef didn't have permission to come before Par'oh when he was still tamei meit before he had buried his father. [In Bereishit 50:4 Yosef lays a petition before the court of Par'oh rather than going directly to Par'oh]

ויעשו כן כאשר צוה, They did as G'd had commanded, etc. The reason the Torah writes the words כן כאשר, i.e. repeating basically the same word is, because it tells us that Moses and Aaron a) did what G'd had told them to, b) did not do so until Pharaoh had requested the miracle from them.