grammar.clauses.noun-clauses
Que-clauses as objects: creo que, dijo que
Que introduces 'that' clauses and is never dropped: creo que tienes razón, dijo que venía. Mood depends on the main verb (assertion vs trigger).
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Explication
A que-clause can serve as the object of a verb — the 'that' clause of English: creo que tienes razón, sé que es tarde, dijo que venía. Unlike English, Spanish never drops que: 'I think you're right' must include it (creo que…).
The mood inside the clause depends on the main verb. Verbs of assertion and perception (saber, creer, ver, decir as 'report') take the indicative: sé que es tarde, veo que funciona. Verbs of wanting, emotion, doubt, and influence trigger the subjunctive: quiero que vengas, dudo que sea así — the subjunctive-trigger rules cover which is which.
Two errors to avoid at the join: dequeísmo, adding de where it doesn't belong (*pienso de que), and queísmo, dropping a required de (*me di cuenta que). The verb decides: pensar que and creer que take bare que, but darse cuenta de que and acordarse de que keep the de.
Exemples
I know the procedure is slow.
Région: global
She told me she was arriving Thursday.
Région: global
I see that you understood, but I doubt it's enough.