"Hace doce dias que..."
"it has been 12 days since I arrived here"
I know this may sound stupid, but after the que, should llegar be in the present or preterite? Thanks!
It depends what you want to say.
"Hace doce días que etoy aquí". I have been here 12 days.
"Llegó hace doce días." - I arrived 12 days ago.
I don't think you can say "Hace doce días QUE llegó.", because with the "que" it refers to a length of time (how long you have been doing something), as opposed to how long ago you did it.