Learn Russian: Они спорят о спорте, а не о политике. - They are arguing about sport, and not about politics.

Because they follow the preposition о (about), which requires the Prepositional case to show the topic of speech. For most masculine nouns ending in a consonant like спорт, and feminine nouns ending in -а like политика, the Prepositional case is formed by adding or changing the ending to -е (making them спорте and политике).

Why do we use о here instead of об?

You use об when the following word starts with a vowel sound (like об Анне - about Anna). Because both спорте and политике start with consonant sounds, we use the standard о.

Why does the sentence use а to mean "and"? Couldn't we use и?

In Russian, а is used to show contrast. It translates to "and" or "but" when you are contrasting two things against each other. Here, it contrasts arguing about sport versus politics. If you used и (the standard "and"), it would mean they are arguing about both topics, rather than one instead of the other.

How do I know спорят means "are arguing" and not just "argue"?