I’m studying about phrasal verbs and I had a doubt:
See those cases:
I picked the children up at school before 6.
I picked up the children at school before 6.
I picked them up at school before 6.
I picked up them at school before 6.
The three ones are okay, but the last one isn’t.
My question is: if I can use ‘pick up’ separeted (sentence 2), why can’t I do this in the 4th sentence?
With the phrasal verbs, when you use a pronoun, it can’t go after the entire verb.
I looked up the word.
I looked the word up.
I looked it up.
It’s fine with regular nouns.
Hi, Barb
I’m afraid, the matter is even more complicated. Having studied phrasal verbs for quite some time, I know that phrasal verbs mainly fall into 2 categories: the ones that can’t have a pronoun inserted between the verb and the preposition, and the ones that can. 
For example, we can say Soon we will be celebrating your birthday. I’m looking forward to it, can’t we?
Or: Life is not fair, get over it.
Or: I hate to go to the dentist for a check-up tomorrow, but I prefer to go there and get it over with, than to postpone it.
Which are also known as separable and inseparable phrasal verbs. 
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You’re exactly right, Amy ! I just forgot how we call them.
The trickiest part is to commit to memory which is which 
I’m not aware of any way to tell “from looking” which type they are – I’m afraid you do have to memorize them. I wish I had a better answer.