Hello everybody! This is a technical question. I am looking into phrasal verbs and there is something I cannot totally catch. Some textbooks of mine explain that when phrasal verbs are followed by a preposition, they cannot be separated. Instead, when they are followed by an adverb, they can be either separated or not. Example: He keyed the number in He keyed in the number So, in the example, "in" is considered an adverb, but it can also be a preposition in English. The same as "after" (to take after). That's the point, it's not easy for me to understand when a particle is an adverb and when it is a preposition. Quite a crazy question, I know, anyway, could you help me figure this out?