From the mailbox (edited for brevity):
Why is this sentence in the subjunctive? Please help me understand this. Cynthia no creía que pudiera meterse nada en el estómago para desayunar. I understand that with "no" pensar/creer uses the subjunctive. But I thought that you needed a subject change. But with Cynthia being the same subject in both clauses, this confuses me.
Welcome to the pitfalls of using subjunctive mood! I think that the source of your confusion lies in thinking that all verbs in this type of sentence must use an infinitive when there's no change of subject. That is true with some verbs, such as querer. If you want to say "I want to leave," then you'd use the infinitive for "to leave": Quiero salir, where salir is the infinitive. But if you want someone else to leave you'd have to use the subjunctive: Quiero que salga, where salga is in the subjunctive. But the verbs where you must use the infinitive when there's no change in subject are few in number and tend to be verbs that relate to the influencing of someone's actions.
But in your sample sentence (which means "Cynthia didn't think she could put anything in her stomach for breakfast"), the main verb relates to mere belief, not influence. So use of the subjunctive here is perfectly fine. In fact, similar uses of the subjunctive are very common after no creer: No creo que yo pueda hacerlo. I don't believe I can do it. Ella no cree que tenga enemigos. She doesn't believe she has enemies.
I hope this helps!