Some phrasal verbs are separable, some aren't. There are no rules - you just have to learn them individually, and remember examples of complete phrases. As native speakers, we all know instinctively what 'sounds' right, and you've obviously developed this instinct because you know what sounds wrong! There is one rule I can give you, though. If you are using a pronoun with a separable phrasal verb, the pronoun has to go between the verb and the particle. The pronoun can't go at the end - that's why 'ask out her' sounds so bad. For example: Take the rubbish out ---> Yes Take out the rubbish ---> Yes Take it out ----> Yes Take out it ---> NO