"Mom wants to leave dad."
Translation:Mamma vil forlate pappa.
Because 'ville' is an auxiliary verb, so 'forlate' follows without the infinitive marker (å). The fact that English uses it is because we use the word 'want' (because our own 'will' took on a different, more specific meaning), this means that any verb which follows uses 'to'. In short - because it's an auxiliary verb and what is true for English isn't true for Norwegian.