1)
Why did Rivkah change the expression from "Lalin" to "Lalun"?
1.
Rashi: In response to Eliezer's request for one night's lodging ("Lalin"), Rivkah responded with "Lalun" - lodging for many nights. 1
2.
Rashbam: "Lalin" is a noun, "Lalun", a verb. 2
3.
Hadar Zekenim: Eliezer requested lodging for many nights. Rivkah responded "Lalun" - for one night, for her father's house was full of idolatry (one should not delay there).