Greetings

Greetings are very important in Japanese. Both greeting and parting phrases are called aisatsu. Someone who makes no aisatsu may be seen as cold and dysfunctional. These aren't all the aisatsu out there, but they'll give you a good start.

Good morning. (Hi.)	Ohayou gozaimasu.
Good afternoon. (Hi.)	Konnichi wa.
Good evening. (Hi.)	Konban wa.
Good night. (Said before bedtime.)	Oyasuminasai. (Lit. Have a good rest.)
It's been a long time.	Ohisashiburi desu.
How do you do? (Said when meeting someone for the first time.)	Hajimemashite.
I ask that you treat me kindly. (This is said often when you meet someone for the first time or when you've asked them to do you some favor.)	Douzo yoroshiku onegaishimasu.
Are you well?	Ogenki desu ka.
Yes, I'm fine.	Hai, genki desu.
Goodbye.	Sayounara./Sayonara. (Sayonara is not normally used when leaving one's own home unless one is leaving for a very long time.)
See you.	Dewa mata./Ja mata./Mata ne.
See you tomorrow.	Mata ashita.
I'm going but I'll be back. (Said when leaving home.)	Ittekimasu.
Have a good time. See you later. (Response to Ittekimasu.)	Itterasshai.
Take care. (Be careful)	Ki o tsukete.
I'm home.	Tadaima.
Welcome home. (Said in response to 'tadaima.')	Okaerinasai.
I'm sorry for leaving before you. (Said as one leaves the office if leaving before other people.)	Osaki ni shitsurei shimasu.
Thanks for working so hard. (Said to someone who's finished a task or in response to Osaki ni. )	Otsukare sama deshita.
Welcome.	Youkoso.
Sorry to bother you. (Said when entering someone's home.)	Ojamashimasu.
Please come in. (Lit. Please come up. Said to someone entering your home.)	Oagari kudasi.