"te" and separable verbs
In which cases does separable verb split into parts when used with "te"?
1) "Hoelang duurt het om X op te lossen?"
2) "Hoelang duurt het om X te voorkomen?"
"Oplossen" is separated, but "voorkomen" is not.
"Voorkomen" is not a separable verb, at least not when it means to prevent. It can also mean to occur/exist, in which case it is separable and is pronounced differently (with stress on "voor"). This is a pretty weird exception, to be honest.
A separable verb is always split when used with "te". For example, using the other meaning of "voorkomen":
- "Deze planten lijken niet in Nederland voor te komen." = These plants do not appear to grow/occur in the Netherlands.
See also: https://en.wiktionary.org/wiki/voorkomen