English: Japanese: Said when:
I'll go and come back Ittekimasu Leaving
Go and come back safe Itterasshai To person leaving
I'm back Tadaima When getting back
Welcome back Okaerinasai When the person gets back
Thanks for the meal Itadakimasu Before a meal
That was delicious Gochisosamadeshita After a meal
I'm here to annoy you Ojamashimasu When entering someones house
Sorry for disturbing you Ojamashimashita When leaving someones house
Please come in Oagari Kudasai When a guest arrives
Thanks Domo Showing gratitude
Thank you Arigato Showing gratitude
Thank you very much Domo Arigato Showing gratitude
Thank you very much (respectfully) Domo Arigato Gozaimasu Showing gratitude
There is one trick that I found really helps when learning these words and that tip is to break the words down. This can help a lot because it makes words seem shorter here is an example of breaking words down.
Ittekimasu= Ittek Imasu
Okaerinasai= Okaeri Nasai
Ojamashimasu= Ojama Shimasu
Finding words that are similar to the words your learning also helps for example one that helped me learn Ojamashimasu and Ojamashimashia was thinking ojama sounds like Obama and the beginning of itadakimasu sounds like eat a.