You were/was

Hello there,

I used to study grammar when i was in middle school. One thing i still clearly remember is that we were taught to use You was .... (only in past tense). But now when i use lots of English books i started coming across with You were a lot. I am just wondering why were taught to use you was then.

I understand it is a plural form and it is supposed to use you were.

At first i thought that using you were was related to smn you don't know well or elder;y, and you was with smn you know well or friends.

Now i don't think so. You were is used which is related to everyone.

Could you explain why you was then?

Thanks.