Learn Russian: Гарантия на этот прочный смартфон стоит тридцать долларов. - The warranty for this durable smartphone costs thirty dollars.

In Russian, certain nouns pair with specific prepositions. When talking about a warranty, a ticket, or a price "for" something, you must use на followed by the accusative case (like гарантия на), rather than using для.

If на takes the accusative case here, why does этот прочный смартфон look like it is in the nominative case?

It is actually in the accusative case! For masculine inanimate nouns like смартфон (and their modifying adjectives), the accusative form is identical to the nominative form. If it were an animate object like a person or animal, the endings would change.

The verb стоит can mean either "costs" or "stands". How do we know which one it is here?

You can tell by the context and the spoken word stress. Here, it means "costs" and the stress falls on the first syllable (стóит). When the verb means "stands", the stress falls on the last syllable (стои́т).

Why is the word for dollars (долларов) in the genitive plural?

In Russian, numbers ending in 5 through 9, the teens (11-19), and round tens like 20 or 30 require the following noun to be in the genitive plural. Because the price is exactly тридцать (30), the masculine noun доллар takes the genitive plural ending -ов.