Learn Portuguese (Portugal): Qual é o rio mais longo do mundo? - Which is the longest river in the world?

In English, we add "-est" to make "longest". In Portuguese, you use the relative superlative structure: o or a + noun + mais + adjective. So, "the longest river" translates to o rio mais longo (literally, "the river most long").

Why does the sentence use do mundo instead of a word for "in"?

When using the relative superlative (like "the longest" or "the biggest"), Portuguese uses the preposition de (of) to introduce the group or place. Since mundo is masculine, de combines with the article o to become do. You literally say "the longest river of the world".

Why use Qual here instead of O que?

Qual is used when you are asking to identify a specific thing from a group or category (like picking one specific river out of all the rivers). O que is used when asking for a definition or a broad, general "What".

Can I change the word order to o mais longo rio?

While you might see it in literature, it is much more natural to say o rio mais longo in everyday speech. In Portuguese, the adjective usually follows the noun, and this stays true in the superlative form: article + noun + mais + adjective.