I thought if the two clauses in a sentence that normally takes the subjunctive both have the same subject, you would use an infinitive form rather than a subjunctive?
I imagine you are referring to this sentence "No creo que yo sepa nada hasta las diez. (I don't think I will know anything until ten o'clock.)". The need of an infinitive when it is the same subject in both clauses doesn't apply to all cases. That rule generally applies to verbs that express "wish or objectives". For example: "Yo quiero ir a Valencia" (I want =subject I, to go=subject I). In this case it would be incorrect to say "Yo quiero que yo vaya a Valencia".
Plus the presence of "que" will be an indicator to use subjunctive. You can only use the infinitive straight after the verb: "Yo quiero ir" not "Yo quiero que ir".
The sentence that I think you are referring to, mentioned above, doesn't talk about "wish", and the only way to use an infinitive there would be without "que": "No creo saber nada hasta las diez." although it sounds more natural to use the subjunctive with "que" as in the original sentence.
I hope this clarifies your doubts.
Thanks for your query, and if you need more clarification, let us know.
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