The imperfect tense of regular (ar) verbs takes place by substituting the ar ending with aba, abas, aba, ábamos, abais, aban.
For example 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 some example sentences:

Pensaba pasar una temporada en Madrid.
I was thinking to spend a spell in Madrid.

Yo siempre andaba en bicicleta cuando era joven.
I always used to ride a bike when I was young.

Tú caminabas en la montaña.
You were walking on the mountain.

Mientras pescaban hablaban del partido de fútbol.
While they were fishing, they were talking about the soccer game.

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

Primero nos tomábamos una copa y después cenábamos.
First we would have a drink and then we would have dinner.

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