auftreten
to perform
To appear in front of an audience as an artist or speaker and give a performance.
Die Band wird nächsten Monat bei einem großen Festival auftreten.
The band will perform at a big festival next month.

to appear / occur
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to perform
To appear in front of an audience as an artist or speaker and give a performance.
Die Band wird nächsten Monat bei einem großen Festival auftreten.
The band will perform at a big festival next month.

'Auftreten' is a separable verb. In a main clause, the prefix 'auf' goes at the end. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Der Schmerz tritt immer morgens auf.
The pain always occurs in the morning.
The perfect tense of 'auftreten' is always formed with the auxiliary verb 'sein' (to be). This is because it describes a change or an appearance.
Die Sängerin ist gestern zum ersten Mal in Berlin aufgetreten.
The singer performed in Berlin for the first time yesterday.
The verb consists of 'auf' (on/up) and 'treten' (to step). The literal meaning 'to step on something' helps you understand the other meanings: you step onto a stage ('auftreten' = to perform), a problem occurs (appears), or you behave in a certain way.
Er ist bei der Konferenz sehr souverän aufgetreten.
He behaved very confidently at the conference.
In the meaning of 'entstehen' (to arise) or 'vorkommen' (to occur), 'auftreten' is often combined with negative or neutral phenomena. Typical examples are: problems, difficulties, errors, symptoms, or doubts occur.
Bei der Installation der Software sind einige Fehler aufgetreten.
Some errors occurred during the installation of the software.
When 'auftreten' means 'sich verhalten' (to behave), it is almost always accompanied by an adverb that describes the manner. Examples are 'sicher auftreten' (to appear confident), 'freundlich auftreten' (to come across as friendly), or 'aggressiv auftreten' (to behave aggressively).
Trotz ihrer Nervosität ist sie sehr selbstbewusst aufgetreten.
Despite her nervousness, she behaved very confidently.
The literal meaning 'mit dem Fuß auf etwas treten' (to step on something with one's foot) is usually recognized from context that refers to the body, a movement, or an injury. It is less common than the other meanings.
Nach der Operation durfte ich zwei Wochen lang nicht auftreten.
After the operation, I wasn't allowed to put weight on my foot for two weeks.
to happen / pass
'passieren' and 'auftreten' can both mean that something happens. 'passieren' is very general and fits many everyday events, especially when something happens unexpectedly or by chance. 'auftreten' sounds more specific and is often used for problems, errors, symptoms, illnesses, or certain phenomena. Learners may confuse them because both have to do with events. As a simple rule: an accident or a story 'passiert', while a problem or a symptom 'tritt auf'.

Wenn nach der Impfung Fieber auftritt, rufen Sie bitte die Ärztin an.
If a fever occurs after the vaccination, please call the doctor.
Gestern ist auf dem Heimweg ein kleiner Unfall passiert.
Yesterday, a small accident happened on the way home.