Learn Spanish (Latin America): La sorpresa es un favor para mí.

Because it comes right after the preposition para (for). After prepositions like para, de, and a, we use special prepositional pronouns instead of our normal subject words. For 'me', that pronoun is mí.

Why does mí have an accent mark?

The accent mark is just there to show the difference between mí (me) and mi (my). Because of the accent, we know this sentence means 'a favor for me' and not 'a favor for my...'.

Why does this sentence use es instead of está for 'is'?

We use es (from the verb ser) to identify or define what something is. Here, we are stating that the surprise equals a favor. When you say 'X is Y', you almost always use ser.

Favor doesn't end in '-o'. How do we know it uses un instead of una?

Many Spanish words that end in consonants don't have an obvious gender rule, so you just have to learn them as you go! Favor happens to be a masculine noun, so it always pairs with un and el.