To combine the sentences into a simple sentence, we can use a subordinating conjunction like "that" to connect the two clauses:\n\nDirect Sentences: He was so tired. He could not run. \n\nSimple Sentence: He was so tired that he could not run. \n\nExplanation: The two independent clauses are joined with the conjunction "that," making the sentence simple while conveying the same meaning. \n\n\n\n \\boxed{\\text{He was so tired that he could not run.}}