sich freuen auf / über

The verb sich freuen uses the accusative reflexive pronoun and usually needs a preposition to say what the joy is about. With auf + accusative, the joy points to the future: you look forward to something that has not happened yet, like Ich freue mich auf das Konzert. With über + accusative, the joy is about something that has already happened or is there now, like Er freut sich über das Geschenk.

The reflexive pronoun changes with the person: ich freue mich, du freust dich, er/sie freut sich, wir freuen uns. A common mistake is mixing up the prepositions, for example Ich freue mich über meinen Urlaub when the holiday is still in the future. Another one is dropping the reflexive pronoun and saying Ich freue das Konzert, which is ungrammatical. The verb can also take a dass-clause or an infinitive, but here we practise the two prepositions.