There are (too?) many ways to say “thank you” in Japanese.
The difference depends on the degree of politeness.
Depending on how much polite you are.
Most polite;
Hontouni Arigatougozaimasu(with bowing)
＝Thank you (very) very much (sir or maam) I really appreciate it.
Arigatougozaimasu.
＝Thank you very much.(sir or maam)(to your boss or customer)
Arigatou.
＝Thank you (to your friends or child)
Sankyuu.
＝Thank you (to your friends or child )
Doumo
＝Thank you (flatly)
(I have read that a Japanese-language guide tells this phrase “Doumo” is very useful. But I do not suggest you use it very often. In my opinion, it does not sound polite at all. Presumably some Japanese people feel annoyed when they hear your “doumo.” )
Lately Japanese people do not say Arigatou and not express their appreciation to others ( or spouses, family members) as often as we are supposed to do. Therefore, non-Japanese people’s Arigatou would remind Japanese people of the importance of expressing their appreciation.
My recommendations:
To your friends or family members: Arigatou
To someone you are NOT so familiar with: Arigatougozaimasu
