Final answer:
In zero conditional sentences, we use the present simple tense in both the if-clause and the main clause. In this case, the sentence would be: 'When Stan finishes his homework early, he plays video games.'
Step-by-step explanation:
In zero conditional sentences, we use the present simple tense in both the if-clause and the main clause to express a general truth or scientific fact. In this case, we can say:
When Stan finishes his homework early, he plays video games.