spontan
spontaneous
Happening out of a sudden impulse and without prior planning or consideration.
Gestern Abend haben wir uns spontan entschieden, ins Kino zu gehen.
Yesterday evening, we spontaneously decided to go to the cinema.

spontaneous
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spontaneous
Happening out of a sudden impulse and without prior planning or consideration.
Gestern Abend haben wir uns spontan entschieden, ins Kino zu gehen.
Yesterday evening, we spontaneously decided to go to the cinema.

The adjective 'spontan' (spontaneous) belongs to a word family. The related noun is 'die Spontaneität' (spontaneity) and describes the quality of being spontaneous.
Ich bewundere deine Spontaneität.
I admire your spontaneity.
When an action happens without planning, 'spontan' (spontaneously) is used like an adverb. It then usually appears in the middle of the sentence and gives more detail about the verb.
Wir haben uns spontan für einen Ausflug ans Meer entschieden.
We spontaneously decided to take a trip to the seaside.
To characterize a person, 'spontan' (spontaneous) is used as an adjective with the verb 'sein' (to be). It then describes a personality trait.
Meine Freundin ist sehr spontan und immer für Abenteuer zu haben.
My friend is very spontaneous and always up for adventures.
You will find this meaning almost exclusively in scientific or medical texts. Typical combinations are 'spontane Heilung' (spontaneous healing), 'spontane Mutation' (spontaneous mutation), or 'spontane Reaktion' (spontaneous reaction).
Die Ärzte konnten sich die spontane Heilung des Patienten nicht erklären.
The doctors could not explain the patient's spontaneous healing.
suddenly
'plötzlich' and 'spontan' can both have to do with something that was not planned far in advance. But the difference is important: 'plötzlich' describes how an event happens, namely quickly and unexpectedly. 'spontan' is more likely to describe a decision, an action, or a person who acts in the moment. Often, a will or impulse is involved. Learners may confuse the words because both happen without much preparation. For an unexpected event, you usually say 'plötzlich'; for an unplanned decision or a flexible person, you say 'spontan'.

Nach der Arbeit gehen wir spontan noch zusammen essen.
After work, we spontaneously go out to eat together.
Beim Einkaufen wird es plötzlich sehr laut im Geschäft.
While shopping, it suddenly gets very loud in the store.