Cuando + El Imperfecto de Subjuntivo (When + the imperfect subjunctive)

Cuando can be followed by the indicative or the subjunctive. It depends on what we want to express. To learn about the use of cuando for habitual or future actions see:

In this lesson we will explain when to use cuando with El Imperfecto Subjuntivo, preceded by a main clause using El Condicional.

Here are some examples:

Yo te ayudaría cuando tú empezaras a comportarte bien.I would help you if l[lit:when] you started to behave.

Estaría bien salir a dar una vuelta cuando dejara de llover.It'd be nice to go for a walk if [lit: when] it stopped raining.

What is expressed after cuando is something the speaker sees as a possible thing to happen but with not much certainty, nearly as a conditional sentence with if

Cuando + Presente de Subjuntivo or Imperfecto de Subjuntivo

When we use cuando with El Presente de Subjuntivo, the speaker sees the action as something with more probability to happen. The level of certainty is higher.

See the difference here:

Te ayudaré cuando te portes bien.
I will help you when you behave.

In the example above the speaker believes it is possible he will behave.

Te ayudaría cuando te portaras bien.
I would help you if [lit:when] you behaved well.

Here the speaker isn't very certain about him behaving. It is more hypothetical.

Now, have a look at these other examples, but this time what's expressed after cuando is simply a future action. Using El Condicional in the main clause triggers (as the most natural tense to use) El Imperfecto Subjuntivo:

Ofrecerían más descuentos cuando pasara la temporada de invierno.They might offer more discounts when the winter season was over.

Pararíamos para descansar cuando llegáramos a Toledo.We might stop for a rest when we arrived in Toledo.

 

In reported speech

We frequently use this construction with cuando followed by El Imperfecto Subjuntivo in reported speech (saying someone said that s/he would do something when something else happened). Have a look at the example:

Me ha prometido que me llamaría cuando aterrizara el avión.He promised he'd call me when the plane landed.

The tense used after cuando is determined by the conditional used within the sentence expressing a reported speech: 

Direct speech:

Te prometo que te llamaré cuando aterrice el avión.
I promise I will call you when the plane lands.

Indirect speech:

Me ha prometido que me llamaría cuando aterrizara el avión.He promised he'd call me when the plane landed.

Important note:

It is important to remember that using

has often a similar meaning to a subordinate clause with "si" (if).  Translating this use of cuando with "when" would be ungrammatical in English, but it is correct to use cuando with this meaning in Spanish. 

Ella cubriría todos los gastos cuando él no pudiera pagar su parte. 
She would cover all costs when if he couldn't pay his part.

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Examples and resources

Ofrecerían más descuentos cuando pasara la temporada de invierno.They might offer more discounts when the winter season was over.
Estaría bien salir a dar una vuelta cuando dejara de llover.It'd be nice to go for a walk if [lit: when] it stopped raining.
Me ha prometido que me llamaría cuando aterrizara el avión.He promised he'd call me when the plane landed.
Yo te ayudaría cuando tú empezaras a comportarte bien.I would help you if l[lit:when] you started to behave.
Pararíamos para descansar cuando llegáramos a Toledo.We might stop for a rest when we arrived in Toledo.
I'll be right with you...