Preposition " a"
  "A" is a preposition generally meaning to or at, with the following uses:
  Ella va a traer el espejo. She is going to bring the mirror
  El tren llega a las doce y media. The train arrives at 12:30
  Vamos a la playa. Let's go to the beach.
  Voy a California. I'm going to California
  It should also be noted that:
  Preposition a contracts to form al when followed by the determinate masculine article el. (a+el)
  This rule can be broken if the article is part of a title
  Voy al país de mis sueños. I am going to the country of my dreams.
  Lo voy a mandar a El Dia. I am going to send it to El Dia (an important Argentinian newspaper).
  The preposition "a" contracts with the definite article "el".
  a + el = al
  Fuimos al centro.  We went downtown.
  Fernando fue al dentista Fernando went to the Dentist
  Llevé las cartas al correo I took the letters to the post office
  The contraction "al"  followed by an infinitive like…

( caminar, comenzar, cerrar, pescar, etc) is a  way of indicating when something happens or is doing.

"Al"  followed by an infinitive is usually the approximate equivalent of "upon," "on" or "when"

  Al :  indicates a temporal value
  Lets see  some examples :
  Comienzan a pescar al encontrar el cardúmen. They start fishing upon finding the school of fish.
  Hay demoras al enviar la mercadería desde ese puerto. There are delays when sending merchandise from that port.
  Cierren la puerta al salir. Close the door on leaving.
  Al parecer, son todas iguales . Apparently, are all the same.
  Me puse  nerviosa al encontrarme con ese viejo amigo. I became nervous upon meeting that old friend.
  Puse mucho cuidado al ordenar su cuarto. I took great care on arranging his room.
   In this case the preposition has no translated value in English.

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