"It was a strange friendship."
Translation:To była dziwna przyjaźń.
I think, that when you can replace "it" with 'this/that" it translates to "TO", if you can replace it with a noun it= on/ona/ono depending on noun (and in declension). if you cant replace "it" with either , then you skip it.
(it's just a theory I made up, so may not be 100% accurate)