The Zero Conditional

The zero conditional is used for when the time being referred to is “now or always” and the “situation is real and posible”. The zero conditional is often used to refer to general truths. The tense in both parts of the sentence is the simple present. In zero conditional sentences, the word "if" can usually be replaced by the word "when" without changing the meaning.

IF CLAUSE	MAIN CLAUSE
If + simple present	simple present
If this thing happens	that thing happens.
If you heat ice	it melts.
If it rains	the grass gets wet.

Zero Conditional (Structure)

More examples:

• If you freeze water, it turns into ice.
  Si congelas el agua, se convierte en hielo.
• If I work too much, I get tired. 
  Si trabajo demasiado, me canso.
• If I have time, I usually go to the movies.
  Si tengo tiempo, generalmente voy al cine.
• If she eats hamburgers, she gets an allergy.
  Si ella come hamburguesas, le da alergia.
• If they come here, they always bring a present.
  Si ellos vienen aquí, siempre traen un regalo.
• If she doesn't know the answer, she keeps silent.
  Si ella no sabe la respuesta, se mantiene en silencio.
• If we don't go out on Saturdays, we rent a video and stay home.
  Si no salimos los sábados, alquilamos un video y nos quedamos en casa.

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Reinforces what was learned:

http://www.aprenderinglesrapidoyfacil.com/2014/01/28/ejercicios-sobre-el-condicional-cero-zero-conditional-exercises/