Lets begin with the past tense of sein which in English means to be
|Pronouns||to be||to be (past)||Pronomen||sein||sein (past)|
|he, she, it||is||was||er, sie, es||ist||war|
As you can see from this chart the form of sein in the past is a form of waren
Here are some examples of how it works:
Ich bin alt aber ich war jung. – I am old but I was young.
Ich war im Kino. – I was at the movie.
Ich war in der Park – I was in the park.
Here is a video of a native German slowly teaching how to conjugate the past tense of sein.
This is the simplest way of making past. Now for some comics illustrating this!