'I know he's coming' is certain, but 'I hope he comes' lives in doubt and desire — and Spanish switches verb form to mark that shift. The subjunctive is the grammar of wishes, doubts, and things that might not be real.
Mood, not tense
The subjunctive is not a tense but a mood — a way of framing a verb as wished-for, doubted, or hypothetical rather than stated as fact. Spanish uses it far more than English, especially in subordinate clauses introduced by 'que'.
Wishes, Emotions, Impersonal expressions, Recommendations, Doubt, and Ojalá commonly trigger the subjunctive: 'Espero que vengas' (I hope you come).
Indicative fact: 'Sé que está aquí' (I know he is here). Subjunctive doubt: 'Dudo que esté aquí' (I doubt he is here).
Drill the trigger phrases until the mood shift becomes automatic.
- Read 'Creo que ___ (venir) mañana'.
- Decide whether 'creer' (to believe) asserts a fact or a doubt.
- Choose indicative 'viene' or subjunctive 'venga' accordingly.
What you should see: 'Creer' in the affirmative asserts belief as fact → indicative 'viene'. Negate it ('No creo que venga') and doubt appears → subjunctive.