Learn Spanish (Latin America): Esas aceras eran estrechas.

Eran is the imperfect form of the irregular verb ser (to be). We use the imperfect here because it describes an ongoing physical trait in the past without a specific start or end time. If we used the preterite fueron, it would imply the sidewalks were only narrow for a specific, completed time frame, which usually doesn't make sense for a physical street description.

Why do we use a form of ser (eran) rather than estar (estaban) here?

A common rule of thumb is that inherent qualities and physical characteristics always use ser. Even in the past tense, being narrow is a built-in physical trait of these sidewalks, not a temporary condition or a location, so we must use ser (eran).

Why is it esas and not esos or aquellas?

Esas means "those" and is the feminine plural form, which matches the feminine plural noun aceras (sidewalks). You would use aquellas (those over there) if the sidewalks were further away from both the speaker and the listener. Esos would only be used if the noun were masculine.

Notice how every word in this sentence ends in "-as" except the verb! Why is that?