Learn Spanish (Latin America): Usted vive en un planeta grande.

Planeta is one of those rule-breaking words! Even though it ends in an a, it is actually a masculine noun. That is why we use the masculine word for a (un) instead of the feminine una. You just have to memorize this one as a special exception: el planeta.

Why does the sentence use en to mean on?

In Spanish, the word en does double duty! It translates to both in and on in English. So, en un planeta works perfectly for on a planet.

Do I need to change grande to match the masculine noun planeta?

No! Adjectives that end in the letter e (like grande, inteligente, or interesante) are exactly the same for both masculine and feminine nouns. Since it does not end in an o or an a, you do not need to change its ending.

Why does Usted use vive? Is that not the form for he or she?

Yes, it is! Usted is the formal way to say you, but it always borrows the verbs used for he and she (él and ella). So, to say you live formally, you use Usted vive, just like you would say él vive (he lives).