Final answer:
To make the sentence correct, a comma should be added after 'soccer' to separate the main clause from the dependent clause.
Step-by-step explanation:
To make the sentence correct, a comma should be added after 'soccer' to separate the main clause from the dependent clause. The corrected sentence would be: 'Many European and South American countries dominate in soccer, although other countries, such as the United States, are improving.'
Learn more about Punctuation in English sentences