In Spanish, the use of a preposition to introduce a subordinate clause serving as an object is possible under certain conditions, such as the presence of a prepositional verb in the matrix clause. As regards to the use of de, studies on queismo and dequeismo discuss a number of usage errors. At the same time, a comparison of the lists given in prescriptive grammars shows that these lists do not always match up and that the use of the preposition is said to be optional with some verbs. Amongst them, there are however controlled and non-controlled cases. The former can be explained by the same syntactico-semantic hypothesis that applies to the use of the definite article to introduce a subordinate subject clause.
Analogías entre el papel del artículo y de la preposición de, en el caso de las subordinadas sujeto y objeto, en español