How did Pharaoh know Moshe and Aharon's names?
Ramban: He asked them their names, and addressed with respect.
Whom was Pharaoh referring to when said ''L'chu le'Sivloseichem"?
Rashi, Ramban #1 and Rashbam: He meant that they should return home to indulge in their personal work. 1 Shevet Levi was not enslaved.
Ramban #2: Since they came to him accompanied by all the people, Pharaoh simply rejected their request and dismissed them all, ordering the people to go back to the heavy work that they were doing for him. 2
Ramban #3 citing the Ibn Ezra): Pharaoh was telling Moshe, who represented the people) to order them to go back to work.
Rashbam: He meant that they should neither be idle from their personal work nor should they cause others to be idle from his (Pharaoh's) work. Refer to 5:4:3:1, and see also Ramban DH 've'Rashi Piresh'.
Rashi writes that Shevet Levi was not enslaved. What is the reason?
Ramban: It was customary for every nation to designate their sages to teach the people their laws and customs. Consequently, Pharaoh exempted the tribe of Levi (by Divine design) from the slavery.