Yes, in addition: When "creer" is followed by a object or object clause, it needs the "que."
Thus: ¿Puedes creer qué genial es esta serie? No puedo creer que ese equipo nos ganó.
¿Puedes creer que se tatuó la cara?
No puedo creer que estés temprano.
But not in these cases:
¡Solo tienen que creer en sí mismos!
¿Pero quién sería tan cínico de creer eso?
Los británicos habían dejado de creer en su propio imperio.
No puedes hacerme creer
To Hucklebeary: I think that, in this case, "lo" has a neutral meaning and so it refers to the entire meaning of the first sentence. In "Creo que ambas LO son", "lo" has the same meaning of the demonstrative pronoun "esto", which is neutral, as well. Both 'lo' and 'esto' are used do avoid the use of the word 'biólogas'. Conclusion: "creo que ambas son biólogas" = "Creo que ambas LO son" = "Creo que ambas son ESTO". I hope I have helped. Greetings. August 23, 2015.
I just spent a weekend with a native Spanish speaker from Spain. He told me I use pensar too much, most of the time Spanish speakers use creer for "think"; I pointed out that creer creates an element of doubt and is followed by the subjunctive while pensar has an element of certainty (because no matter how much doubt there is about what you think, the fact that you do think is certain). No matter, sez he...use creer most of the time and you will get by. Go figure...
The distinction between thought and belief is a huge one, and the fact that Americans use "think" almost exclusively for both is no small social problem. It can be difficult to communicate to an American that what you "think" actually involved thought, and didn't just sprout from your backside, even harder to convince him that it's important. At first the anecdote of your Spanish friend bothered me, but then I realized the problem would be if "creer" were lost and "pensar" became ubiquitous and diluted. As an American living in Latin America I've observed with interest how other gringos have coped with creer/pensar, and I can promise it's fascinating enough to recommend. (Putting unjustified belief on a level with rational thought and expertise is broadly discrediting to scientists and experts. This issue is studied within epistemology, a branch of philosophy. I recommend to everyone a web search for more information.)
Yes, corroboration is always useful.
In the meantime, let me correct myself: The only direct object ‘creer’ can take is a person:
«Te creo.» = “I believe you.”
«Creo en ti.» = “I believe in you.”
«Creo al testigo.» = “I believe the witness.”
«Creo en el testigo.» = “I believe in the witness.”