
sich beteiligen an
to participate in
To actively take part in a joint action, project, discussion, or event.
Alle Schüler haben sich an der Aufräumaktion beteiligt.
All the students participated in the cleanup campaign.
to participate / involve
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to participate in
To actively take part in a joint action, project, discussion, or event.
Alle Schüler haben sich an der Aufräumaktion beteiligt.
All the students participated in the cleanup campaign.

to give someone a share in
To give another person a share in something, usually in the financial profit or success of a company.
Das Unternehmen will seine Mitarbeiter am Gewinn beteiligen.
The company wants to give its employees a share in the profits.
The verb contains the word 'Teil' (part). The prefix 'be-' and the ending '-igen' turn it into a verb. So 'sich beteiligen' literally means to make oneself part of something.
Er wollte ein Teil der Lösung sein und beteiligte sich an der Diskussion.
He wanted to be part of the solution and participated in the discussion.
This verb has two main patterns: 'sich beteiligen an' (reflexive) means to participate oneself. 'Jemanden beteiligen an' (transitive with an accusative object) means to give another person a share.
Die Bürger beteiligten sich aktiv an der Planung des neuen Parks.
The citizens actively participated in the planning of the new park.
In both meanings, 'beteiligen' is used with the fixed preposition 'an' and the dative. A common mistake is using a different preposition.
Viele Freiwillige beteiligten sich an der Hilfsaktion.
Many volunteers participated in the aid campaign.
to participate / sympathize
Both words can be used for a shared event or activity. „Teilnehmen“ can also mean simply being present and joining in, while „sich beteiligen“ places more emphasis on active involvement. Use „teilnehmen“ for mere attendance and „sich beteiligen“ when someone contributes something themselves.

Mia beteiligte sich an der Diskussion und stellte mehrere Fragen.
Mia took part in the discussion and asked several questions.
Mia nahm an der Diskussion teil und hörte nur zu.
Mia attended the discussion and only listened.