"I was running when I fell over!"

This is the past continuous. It's when something is happening in the past and something else happens.
Here, whilst I was running I fell over.
"What were you doing?" "I was running."

We also use the past continuous when replying to a question that is asked in the past continuous.
I was in the process of running when something else happened. My shoelace came undone, perhaps.
“I was wondering if you could cook dinner tonight.”

Here we use the verb “to wonder” to ask someone politely if they would mind doing something, of if they minded that you do something. Here, I’m asking if you would mind cooking dinner tonight! A barbeque would be nice :)
“Forrest Gump was always running.”
We can also use this tense when something is habitual.
To construct the past continuous, we use the past tense of the verb "to be" together with the verb+ing:-

Nothing out of the ordinary for negative and question constructions. Look:

Now it's your turn, can you complete these sentences?
I (read) Macbeth.
It (snow).
We (make) a cake.
You (ride) my bike.
She (run) fast.
He (paint) his house.
He (write) a book about his life.
Leave your answers in the comments, and if you want more information, just ask.