I was x I were: Qual a diferença

Hello guys!

I've studied English since 2012, but only today I read something that sounds me weird.
I was watching a video on YouTube from a native speakers and she said that in the past continuous or past progressive we use "I was..." ok!But what's the difference between "I was" and "I were" and more, when we have to use them?

thanks in advance for your help!