Swettenham said:
Ah, no conozco este uso de creo + infinitivo. Supongo que quiere decir "Creo que yo entender." Que decis vosotros la gente? (no puedo hoy hacer mis acentos)
Well, I haven't done the pertinent research in a grammar source yet, but from what I remember, you can use a verb in infinitive as a noun, that is, "sustantivar". I think the corresponding grammatical concept in English would be that of using either "to + verb" or "-ing":
(El) olvidar es triste: here, olvidar (in infinitive) is acting as a subject, as a noun.
Forgetting is sad.
So, I suspect that when you use "Creo + verbo en infinitivo" this verbo en infinitivo is acting as a direct object and, therefore as a noun. I'd say that the general rule would be something like if the verb you want to use is acting as a direct object, then you should use it in infinitive.
Another example: Gustar + verbo en inf.:
Me gusta
acampar en la montaña.
¿No recuerdas
haber ido al mar cuando niño?
Ella siempre olvida
cerrar la puerta con llave.
Nosotros siempre queremos
comenzar nuevas discusiones en la mitad de un hilo.
Ellos nos recuerdan
callarnos.
Acampar, haber, cerrar, comenzar and callar(nos) are acting as a direct objects and answer the question ¿qué?