My son brought his Japanese girlfriend home for dinner. After dinner, they went out for a walk. I thought I would try to be friendly to the girlfriend by saying “welcome home” when they come back. How should I say this in Japanese?
Answer by Professional Japanese Teacher
Okaerinasai is a greeting word to welcome people who have come home. The original phrase was よくご無事（ぶじ）に、お帰（かえ）りなさいました Yoku gobujini okaerinasaimashita (You’ve come back safely without any harm) but it was shortened to okaerinasai over time.
Okaerinasaimashita is the honorific past form of the word 帰る kaeru which means “to return home”. You can use okaerinasai for the girlfriend. For family members and close friends, you can also use the shorter and more casual version okaeri.