The imperfect tense of regular -ar verbs is formed by substituting the "ar" ending with aba, abas, aba, ábamos, abais, aban.

As an example, let's consider the verb pensar (to think)

pens + (ending) --> pensaba, pensabas, pensaba, pensábamos, pensabais, pensaban

The first and third person singular forms are identical, therefore the context must distinguish between the two. Let's look at a few sentences using the verb pensar.

Yo pensaba en vosotros.
I was thinking about you guys.

¿En qué pensabas?
What were you thinking about?

Ella pensaba salir de compras.
She was thinking of going shopping.

Marruecos no es como pensábamos.
Morroco is not the way we were thinking it was.

Now, let's review the use of other -ar verbs.

caminar - to walk
Tú caminabas en la montaña.
You were walking on the mountain.

pescar - to fish
hablar - to talk
Mientras pescaban hablaban del partido de fútbol.
While they were fishing, they were talking about the soccer game.

llamar - to call
estar - to be
Siempre me llamaba cuando estaba de viaje.
He always used to call me when he was traveling.

tomar - to take, to drink
Mi gato siempre tomaba leche.
My cat always used to drink milk.