Itadakimasu. () / Gochisousamadeshita. ()
In Japan, it is customary to say “Itadakimasu” before you eat and “Gochisousamadeshita” after you finish. “Gochisou ()” means banquet or delicious meal. You should say “Gochisousamadeshita” (=Thank you for the treat) when someone treats you to a meal.
Both “Itadakimasu” and “Gochisousamadeshita” convey a meaning of thankfulness for the meal. Keep in mind that both of these expressions are used before and after you eat regardless of whether or not someone cooks for you or treats you to a meal.

Ittekimasu. () / Itterasshai. ()
“Ittekimasu” (=I’m leaving now / I’m heading off) is used when you leave your house. Those who see you off would respond by saying “Itterasshai” (=See you / Have a nice day).

Tadaima. () / Okaerinasai. ()
When coming back home, it is conventional to say “Tadaima.” This phrase both means “now” and an abbreviated version of “I’m home” (=Tadaima kaerimashita.). The person at home responds with “Okaerinasai” (=Welcome back / Welcome home).

“Ittekimasu. / Itterasshai.,” “Tadaima/Okaerinasai” are greetings used not only at home, but also in the workplace or when you greet your neighbors.

Ojamashimasu. () / Ojamashimashita. ()
When entering someone’s home, you should say “Ojamashimasu.” When leaving someone’s home, you should say “Ojamashimashita.” The original meaning of “Jama ()” means “to interrupt” or “distraction” but “Ojamasuru ()” is a polite way to say “to visit someone’s home.”