was is a past form of the verb to be. The Japanese equivalents of these verb are arimasu (for nonliving things) and imasu (for living things). (desu is a seperate issue, I think you know about it already, its like an = sign)
Lots of Japanese folk stories start with a line "Mukashi mukashi, aru tokoro ni ~~~~ ga sunde imashita." You can put "otoko no ko to onna no ko" into where I put "~~~".