The imperfect tense of regular -er and -ir verbs take place by substituting the "er" and "ir" ending by ía, ías, ía, íamos,íais, ían.

Let's review the verb vivir (to live) as an example:

viv + (ending) --> vivía, vivías, vivía, vivíamos, vivíais, vivían

The first and third person singular forms are identical, therefore the context must distinguish between the two.

Let's look at some example sentences:

Cuando vivía en París me encontraba con él a menudo.
When I was living in Paris, I meet with him frequently.

Tú vivías enfrente de mi casa.
You used to live in front of my house.

Vivíamos en México.
We used to live in Mexico.

Vosotros vivíais cerca de la entrada a la ciudad.
You all used to live near the entrance to the city.

Ellos vivían en la esquina.
They used to live around the corner.