For "sudden" or "unexpected":
* -aneous: This suffix often implies suddenness or happening without warning, like "instantaneous" or "spontaneous".
* -ic: This suffix can sometimes suggest a sudden or impulsive action, like "fantastic" or "dramatic" (though it doesn't always imply spontaneity).
For "unplanned" or "unforced":
* -less: This suffix can be used to suggest something happening without effort or planning, like "effortless" or "careless". However, this might not always convey the specific meaning of spontaneous.
For "automatic" or "natural":
* -matic: This suffix indicates something that happens automatically, which might be related to spontaneous actions, like "automatic" or "systematic".
It's important to remember that these suffixes don't always carry the exact meaning of "spontaneous" and might need to be used in combination with other words or phrases to convey the desired nuance.
For a more precise and natural expression, you could consider using descriptive words like:
* Impulsive
* Unplanned
* Unprompted
* Instinctive
* Unforeseen
* Sudden
* Unexpected
Ultimately, the best choice depends on the specific context and the meaning you want to convey.