When you greet someone
おはようございます。Ohayō gozaimasu. · Good morning.
This is a polite morning greeting. With close friends or family, you may hear the shorter おはよう (ohayō).
When someone comes home
おかえりなさい。Okaerinasai. · Welcome home.
Say this to someone returning home. The person arriving often replies ただいま (tadaima), meaning “I’m home.” The two phrases make more sense as a pair.
When someone helps you
ありがとうございます。Arigatō gozaimasu. · Thank you.
This is a polite way to express thanks. The shorter ありがとう (arigatō) fits more casual relationships.
Before eating
いただきます。Itadakimasu. · Said before a meal.
This is a customary expression before eating. It is often glossed as thanks for the food, though it does not have a perfect one-word English equivalent.