Phasal verb: pick up - separated

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.

  • I looked up it - No.

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. :slight_smile:
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. :smiley:

You’re exactly right, Amy ! :slight_smile: I just forgot how we call them.
The trickiest part is to commit to memory which is which :wink:

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.