Learn Spanish (Latin America): Con ese millón, compraría una mansión carísima. - With that million, I would buy an extremely expensive mansion.

It is the conditional tense, which translates to "would buy". For regular verbs, you simply take the full infinitive (comprar) and add the ending -ía. Here it means "I would buy", but because the yo, él, ella, and usted forms all end in -ía, you usually rely on context or add a subject pronoun if the person is unclear.

Why does the sentence use carísima instead of muy cara?

Adding the suffix -ísimo or -ísima to an adjective is a very natural way to say "extremely" or "super" in Spanish. You drop the final vowel of cara (expensive) and add -ísima so it correctly matches the feminine noun mansión.

Why is it ese millón and not eso millón?

Ese is the masculine singular word for "that" when it comes right before a noun like millón. Eso is the neuter form, which is only used for abstract ideas, situations, or unidentified objects. You never put eso directly in front of a masculine noun.

How can I remember that mansión is feminine?