sich freuen auf
to look forward to
To feel positive expectation and anticipation for a future event.
Die Kinder freuen sich schon seit Wochen auf die Sommerferien.
The children have been looking forward to the summer holidays for weeks.

to look forward / to be pleased
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to look forward to
To feel positive expectation and anticipation for a future event.
Die Kinder freuen sich schon seit Wochen auf die Sommerferien.
The children have been looking forward to the summer holidays for weeks.

The verb 'freuen' (to be happy / to please) is almost always used reflexively, that is, with 'sich'. The reflexive pronoun must match the person: ich freue mich, du freust dich, wir freuen uns.
Ich freue mich schon auf das Konzert am Samstag.
I'm already looking forward to the concert on Saturday.
The preposition shows what the joy refers to. 'Sich freuen auf' (+ accusative) describes looking forward to something in the future. 'Sich freuen über' (+ accusative) describes being pleased about something present or past.
Ich freue mich auf meinen Geburtstag nächste Woche.
I'm looking forward to my birthday next week.
The non-reflexive form 'freuen' (to please) is often used in formal or polite situations. The person who is pleased is in the accusative, and the subject is often the impersonal 'es' (it).
Es freut mich, Sie kennenzulernen.
It pleases me to meet you.
to hope
'hoffen' and 'sich freuen' both have to do with positive feelings. 'hoffen' is used when something is still uncertain and you want it to turn out well. 'sich freuen auf' fits when you are looking forward positively to a future event, for example a visit or a vacation. 'sich freuen über' is used when something has already happened or is present now. The risk of confusion comes from the fact that you can look forward to something good and at the same time hope that it works out. As a practical rule: when there is uncertainty, 'hoffen' fits; when there is anticipation, 'sich freuen auf' fits.

Nach dem Unterricht hat sich der Mitarbeiter auf das Training im Park gefreut.
After class, the employee was looking forward to the training session in the park.
Wegen des Regens hoffen wir, dass das Fußballspiel trotzdem stattfindet.
Because of the rain, we hope that the soccer game will still take place.