A) Rodrigo extrajo de su bolsillo el boli con que normalmente firma y empezó a escribir
B) Rodrigo extrajo de su bolsillo el boli con el que normalmente firma y empezó a escribir
These two sentences are the same. They express the same thing in Spanish. But for a native speaker, one will probably sound better than the other one (or maybe both will same natural). However, the final conclusion is that both are perfectly correct. In both of them we find a relative clause introduced by a relative group/phrase. This relative group is made up of a preposition ("con" in this case) followed by "que" in A and by a "definite article + que" in B.
Let´s now see these other sentences:
C) Rodrigo trajo consigo una libreta a que apodaba cariñosamente "hija" (INCORRECT)
D) Rodrigo trajo consigo una libreta a la que apodaba cariñosamente "hija" (CORRECT)
On this occasion we have also two sentences with the same characteristics as A and B (a relative clause introduced by a relative group). But this time the first one is not correct and does not sound natural to a native speaker. But, why? Why is "A" correct but on the other hand "C" is not if they have the same pattern (Preposition + que) when it comes to the relative clause?
This is something that creates chaos and confusion among students of Spanish as a foreign language. Generally speaking, the European variety tends to use the article always in these structures but the American variety tends to use it only when it is mandatory/needed. And that is why we can hear examples as "C" coming from students.
The aspects that make mandatory the presence of the definite article in these Spanish structures are mainly four:
1. The preposition
2. The type of relative clause
3. The antecedent of the relative pronoun/group/demonstrative
4. The syntactical structure of the relative clause
I will explain these aspects in the next posts, making two posts explaining two of the aspects in each.