The dual nature of prefixed verbs
In Dutch, verb prefixes like over-, onder-, and door- can be separable or inseparable. The difference lies in word stress and meaning.
Separable: Literal meanings and stressed prefixes
When the stress falls on the prefix, the verb is separable. These verbs usually describe literal, physical movements or directions, and the prefix splits off to the end of a main clause.
When overkomen means "to come over", the stress is on over (óverkomen).
Similarly, when ondergaan means "to go under" or "to set" (like the sun), the prefix is stressed (óndergaan) and the verb separates.
Inseparable: Abstract meanings and stressed verb roots
When the stress falls on the verb root, the verb is inseparable, keeping the prefix permanently attached. These usually have abstract or figurative meanings. Because they never split, their past participles do not take a ge- prefix.
Compare the inseparable overkomen (overkómen), meaning "to happen to":
Notice how the past participle is just overkomen, not overgekomen.
Now compare the inseparable ondergaan (ondergáán), meaning "to undergo":
More abstract inseparable verbs
Other abstract, inseparable verbs include doorzien (doorzíén, to see through) and voorkomen (voorkómen, to prevent):
