Confusing isn't it? You would think it should be "There were" instead of "There was", but it isn't. This is because "There is/are..." is a special English construction called an "expletive constructïon". To get the right verb, substitute "it" for "there" You would not say "Then it were you." - You would say "Then it was you." That's how you get the verb. I'm not sure other languages have a construction like this (exactly). That's why it's so confusing. On behalf of all English speakers, I apologize for the inconvenience.

Thanks. I have thought that the subject is you.

How much I love this song! :o) About the grammar: There was a cat in her room. / There were cats in her room. There was - for one thing or something uncountable There were - for two or more things you = one thing ... Till there was you! :o)