Subjunctive. I'm going to rent them some horses as soon as I arrive at the park. ---- You haven't arrived yet, and you can't be 100% sure you'll ever arrive. -- You could be hit by a bus and die, for example. -- This lack of certainly means you should use subjunctive.
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Subjunctive. I'm going to rent them some horses as soon as I arrive at the park. ---- You haven't arrived yet, and you can't be 100% sure you'll ever arrive. -- You could be hit by a bus and die, for example. -- This lack of certainly means you should use subjunctive.