if - si
whether - si
unless - a menos que
provided / providing - siempre que, mientras
as long as - siempre que, mientras
in case - en caso de que

if - si

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• If I see him, I'll give him the message.
  Si lo veo, le daré el mensaje.
• What shall we do, if they don't accept the offer?
  ¿Qué haremos si ellos no aceptan la oferta?

whether - si

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• I don't know whether they will agree on that subject.
  No sé si ellos estarán de acuerdo en ese tema.
• I wonder whether she really wants to do that.
  Me pregunto si ella realmente quiere hacer eso.

unless - a menos que

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• You won't pass the exam unless you study harder.
  No pasarás el exámen a menos que estudies más duro.
• Don't go there unless they call you.
  No vayas allá a menos que ellos te llamen.

provided / providing - siempre que, mientras

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• We'll go fishing this afternoon, provided it doesn't rain.
  Iremos a pescar esta tarde, siempre que no llueva.
• You can borrow my car providing you return it today.
  Puedes tomar prestado mi coche siempre que lo regreses hoy.

as long as - siempre que, mientras

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• You can stay here as long as you work hard.
  Te puedes quedar aquí mientras trabajes duro.
• Nobody will know this secret as long as you keep mum.
  Nadie sabrá este secreto siempre que mantengas la boca cerrada.

in case - en caso de que

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• Call this phone number in case you get lost.
  Llama a este número de teléfono en caso de que te pierdas.
• We will take our umbrellas in case it rains.
  Llevaremos nuestros paraguas en caso de que llueva.