Some Spanish verbs, adjectives, and expressions always require a specific preposition. When they are followed by a clause starting with que (that), the preposition must stay! Dropping it is a common mistake known as queísmo.
Using "de que"
Verbs like darse cuenta de (to realize), acordarse de (to remember), and olvidarse de (to forget) always need de. If the next word is que, you must say de que. Adjectives like seguro/a de (sure of) work exactly the same way.
| We have realized that the mechanism is broken, since it does not turn. | Nos hemos dado cuenta de que el mecanismo está roto, puesto que no gira. |
| The boy has forgotten that the trapdoor opens with the timer. | El chico se ha olvidado de que la trampilla se abre con el temporizador. |
Fixed expressions like antes de (before) also keep their preposition when followed by a que clause.
| Do not slide the lever before I turn the crank! | ¡No deslices la palanca antes de que yo gire la manivela! |
Using "a que" and "en que"
Other verbs demand different prepositions. For example, esperar a (to wait for an event) and acostumbrarse a (to get used to) become a que.
| Anyway, we wait for the trapdoor to open. | De todas formas, esperamos a que se abra la trampilla. |
Similarly, insistir en (to insist on) and fijarse en (to notice) become en que.
| Having found the exit, I insist that you slide the lever carefully. | Habiendo encontrado la salida, insisto en que deslices la palanca con cuidado. |
The "eso" trick
How do you know if a verb needs a preposition? Try replacing the entire que clause with eso (that).
- Me he olvidado de eso makes perfect sense, so it translates to Me he olvidado de que...
- Creo de eso sounds wrong (it should just be Creo eso), so it translates to Creo que...
Adding an extra de where it doesn't belong is a mistake called dequeísmo. Verbs of thinking and communicating, like creer (to think/believe) and pensar (to think), never take a preposition. They connect directly to que.
| I think that we need the magnifying glass to read the parchment, in any case. | Creo que necesitamos la lupa para leer el pergamino, en cualquier caso. |
| I think that the magnifying glass reveals the symbol of the mechanism. | Pienso que la lupa revela el símbolo del mecanismo. |