There is no merging process involved; if you mean by merging something as in “à le => au”, that is called rather “contraction” and that has nothing to do with whether to use “à la” or “en”; “à” is one preposition used among other things to indicate the place where something is and also the place where someone/something goes and also to give indications of time. “En” is another preposition, and that one is used to indicate basically a place where someone/something is or into which someone/something goes and also to give indications of time. The meaning of this one is comparable to that of “dans”.

A/ à la, à l’

• For nouns that are not the nouns of countries or regions which gender is feminine (Amérique, Allemagne, Belgique,…) one of the two forms is used according as the noun begins with a consonant or not; if the consonant is an h, then you use “à l'” when the h is a so called “h muet”.

  Elle est à la gare. _ Les enfants vont à la gare. _ Ces jeunes sont à l’armée. _ Les jeunes vont à l’armée à dix-huit ans. _ Il a été blessé à la hanche. _ Je te le dirait à la saison des fleurs.

• ready made phrases without the article (no indication of location)

  À (la) seule fin de (the speaker can choose whether to use the article or not)
  Les gens avaient couru à perte de souffle. _ Mettez du grain dans le second plateau de la balance jusqu’à concurrence de trente grammes.

B/ en

• For the nouns that are the names of countries and regions “en” must be used; that is nothing else but a matter of usage; however there are exceptions.

  la France => en France, l’Espagne => en Espagne; Camargue => en Camargue, Île-de- France => en Île-de-France

  à la Martinique, à la Guadeloupe,

• For common nouns there two possibilities to distinguish among others;

  • normal usage with the article; indication of place, and of time but for certain nouns on.

    En la (ou sa) partie supérieure la boite comprend des trous. _ L’explosion la détruisit en son entièreté.
    C’est arrivé en l’année 1959. _ Il les reverrait en la saison du renouveau.

  • ready made phrases without the article (indication of location or otherwise)

    des sardines en boite _ Il y avait sur la table des livres en pile. _ La confiture est mise en pot toute chaude.