LAS LENGUAS INGLESA Y ESPAÑOLA de Delfín Carbonell

Se prefiere a en vez de per , pero se ha afincado esta última palabra mucho en el habla coloquial y a los puristas les repatea su uso y dicen que se debe emplear sólo en cuestiones técnicas como: twenty miles per hour , fifty dollars per day. Podemos decir perfectamente twenty miles an hour , fifty dollars a day.  En frases como percent , per anum, per capita, per sesion  es la única alternativa.  En vez de as usual  oiremos mucho per usual: The team, per usual , was its own worst enemy. "He was eating one grain of rice per day." ( The Buddah , 2010.)

For years I have condemned the use of a por as a solecism. I have always abhored the sentence voy a por la leche . The rationale was that we should not employ two prepositions together, in this case a and por . But language does not belong to stick-in-the mud old fogies like me, luckily. It belongs to all and has its own directions and rules. So I have changed my mind because there is a reason, I admit, for this a por . Voy a misa por mi mujer. (Because she wants me to, and to keep her happy.) Voy a misa a por mi mujer. (To collect her or pick her up.) He venido por ti. (Because of you.) He venido a por ti. (To get you.)