The imperfect tense in Spanish describes actions that were ongoing, habitual, or descriptive in the past. It is used to talk about weather, habitual actions, emotions, age, time, dates for context, and descriptions. The imperfect tense conveys background information and sets the scene. It is formed by adding AR, ER, or IR endings to the verb stem, with yo, tú, él/ella/usted, and ellos/ellas/ustedes taking different endings. Only three verbs, ser, ir, and ver, are irregular in the imperfect.