The use of the imperfect subjunctive depends on the time frame given by the verb in the main clause. Let´s review the following examples:

- Es importante que traigas tu pasaporte.
  (It is important that you bring your passport.)
- Era importante que trajeras tu pasaporte.
  (It was important that you bring your passport.)

  Note how in Spanish the subordinate clause changes according to the tense of the main clause. If the main clause uses the imperfect tense, the subordinate clause must use the imperfect subjunctive.
  The imperfect subjunctive uses the following endings. Note that both forms may be used interchangeably:

   In subordinate clauses we use the imperfect subjunctive as follows:
- With verbs that express wish, fear, hope,etc., in the main clause:
  - Temía que no vinieras. (I was afraid that you would not come.)
  - No pensaba que lo hicieras tan bien. (I didn´t think that you would do it so well.)
  - Me gustó que ayudaras a tu hermana. (I liked that you helped your sister.)

- When the subordinate clause expresses conjectures, something unreal or not previously experienced:
  - No lo haría aunque quisiera. (I would not do it even if I wanted to.)
  - Si eso fuera cierto ya habría sido publicado.
    (If that were true, it would have been published already.)

- As a polite way of expressing desire and with a few constructions:
  - Quisiera saber cómo llegaste a mí. (I would like to know how you came to me.)
  - Quisiera ir contigo. (I would like to go with you.)
  - Dijera lo que dijera, nunca le creerían.
    (Whatever he might say, they would never believe (him/her/you formal).)

- In a few exclamations with no subordinate clause. In this case we use it to express a wish or to stress situations that we considered unlikely to happen:
  - ¡Ojalá me tocara la lotería! (I wish I win the lottery!)
  - ¡Quién lo pensara! (Who would have thought of it!)

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aspects of Spanish grammar.