When to use the "a" after "va"

Question

I know that "va a durar" means "going to last" and "va durar" means "will last". I often get confused as to when I should use the "a" after the "va" or "voy" in a sentence. Can someone please explain the reasons for including and not including the "a" in a sentence after "va"? Thanks!

Answer 1

Use "ir + a + infinitve" when you are expressing that someone is "going to do something".

Ex: Voy a llevar a mi hermana a su casa.

I am going to take my sister to her house.

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"Will last" is durará.

Answer 2

Me fue enseñado que siempre después del verbo Ir se pone la preposición 'a'. Pienso que la manera gramaticalmente correcta sea siempre utilizar el 'a' enseguida del verbo Ir, pero, diariamente los hispanohablantes pueden no utilizar la preposición. No lo sé con certeza.