freiwillig
voluntary
Done of one's own free will and without coercion or payment.
Sie leistet freiwillige Arbeit in einem Tierheim.
She does voluntary work at an animal shelter.

voluntary
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voluntary
Done of one's own free will and without coercion or payment.
Sie leistet freiwillige Arbeit in einem Tierheim.
She does voluntary work at an animal shelter.

The word is made up of 'frei' (free) and 'Wille' (will). To do something 'freiwillig' (voluntarily) therefore means to do it of one's own free will.
Die Teilnahme am Kurs ist freiwillig.
Participation in the course is voluntary.
'Freiwillig' (voluntary / voluntarily) can stand not only as an adjective before a noun, but also as an adverb that describes a verb. It then answers the question 'How?'.
Er hat freiwillig beim Aufräumen geholfen.
He voluntarily helped with cleaning up.
Common combinations are 'freiwillige Arbeit' (voluntary work), 'die freiwillige Feuerwehr' (the volunteer fire department), or the action 'sich freiwillig melden' (to volunteer).
Viele junge Leute leisten einen freiwilligen Dienst im sozialen Bereich.
Many young people do voluntary service in the social sector.
gladly / like to
'gern' and 'freiwillig' can both express that someone does something without being forced. The difference lies in the focus: 'gern' mainly says that you do something with pleasure or enjoyment. 'freiwillig' mainly says that you do something of your own free will, without coercion. You can do something voluntarily even if you do not like doing it. Learners can easily confuse the words because both are often used for actions based on one's own decision. If the enjoyment is important, 'gern' fits better. If the point is that there is no coercion, 'freiwillig' is more precise.

Im Büro hat sie freiwillig beim Aufräumen geholfen.
At the office, she voluntarily helped with tidying up.
Zum Abendessen koche ich gern etwas Warmes.
I like to cook something warm for dinner.