Your question should be, "When do we have to separate phrasal verbs?" This is a lot more complex than you'd like it to be. Some phrasal verbs don't separate, and some do. Some phrasal verbs take an object, but don't separate. You need to learn the rules for each new phrasal verb. If a phrasal verb is separable, you must separate it if you use an object pronoun: me, you, him, her, it, us, them. You can say "take off your hat" or "take your hat off", but you must use "take it off" ("take off it" is simply wrong).

Interesting question. I cannot assure it for you, when you need to separate or not or even if there's a pattern. However, do you know what is an intransitive and transitive verb? An intransitive verb basically doesn't need a object, then, is inseparable. On the other hand, a transitive verb needs an object, therefore, is separable. The problem is: some verbs can be both of them. Like "take off". I think when you face a phrasal verb, find out on a dictionary to see if it is transitive or not. I hope that these tips can help you. Keep studying and good luck!