Fadat besham = ghorbunet beram = doret begardam " I'm the soil underneath your feet" is correct. Setareye soheil shodi= you Daste shoma dard nakone= when someone do a favor for you and you want to thank him or her. Khaste nabashi= literally it means you don't be tired. But it uses when a person just finish a taugh job like washing dishes or teaching students or come back Home from office. It means you care about him or her and give him/her energy.

Some other phrases like ''Ghabeleh shoma ra nadarad" it means " It's worthless for you" For example when you give a gift to someone and they said "kheyli mamnoon"(Thank you), you could say "ghabele shoma ra nadarad". Other phrase is "Bozorgitun ra Miresoonam" for example when somebody say you 'Say hello to your Mom' then you can say "Bozorgituno miresunam" it means that 'I say your Greatness to her'. It's something more than like Hello, for more respect to your addressee.