Learn French: Je lui promets une belle surprise. - I promise him a beautiful surprise.

It could be either! The indirect object pronoun lui means "to him" or "to her". The verb is promettre à (to promise to), which requires this indirect pronoun. You would know who it refers to from the rest of the conversation.

Why is lui placed before the verb promets?

In French, object pronouns like lui almost always go immediately in front of the verb they are linked to. Instead of saying "I promise him" like in English, the French structure is literally "I to-him promise".

Why does the adjective belle come before the noun surprise?

Most French adjectives come after the noun, but short, common adjectives describing Beauty, Age, Goodness, or Size (the BAGS rule) go before it. Because belle refers to beauty, it goes right in front of surprise.

Is promets an irregular verb?

Yes, promets comes from the irregular verb promettre. But there is a great trick to learning it: it follows the exact same pattern as mettre (to put). You just take je mets and add the prefix pro- to get je promets.