Phrasal verbs: five different structures
Anton from Vietnam writes:
  Some grammar books say that in phrasal verbs (for example: look for) the particle is not separated from the main verb. But there are some exceptions, e.g. put the clothes on, put them on. Is put on them also acceptable?

With the look for example, we can say: He is the boy I am looking for. But we can also say: He is the boy for whom I am looking. So the particle can be separated. Please explain.
Roger Woodham replies:
A phrasal verb is a verb that is combined with a preposition (e.g. in, on, with) or an adverbial particle (e.g. up, out, off) The preposition or adverbial particle extend the meaning of the verb to create a new meaning. For example if you look at something, your eyes are focused on it, but if you look after something or someone, you take care of them.

Phrasal verbs follow one of five different patterns.
verb + prep / adv + object
Look for and look after follow this pattern. The verb is followed by a particle and an object. The particle is not separated from the verb. Other common phrasal verbs following this pattern (and there are many of them) include: take after (resemble), fall for (fall in love with), come across (meet by chance):
I'm looking for my glasses. I've been looking for them all day.
Can you look after my cat while I'm away? Ill pay you for looking after him.
I take after my cousin. Everybody says I take after her.
I've really fallen for this guy. Have you ever fallen for anyone?
Did you come across any photos of the family when your were clearing out the attic?
Exceptionally with this pattern, Anton, the particle is separated from the verb when it is combined with a relative pronoun introducing a defining relative clause, as in:
He is the boy for whom I am looking.
This is the relative after whom I take.
But this is an example of very formal English. We would normally say:
He is the boy I'm looking for.
This is the relative I take after.
verb + prep / adv + object OR verb + object + prep / adv
Some phrasal verbs can be used in either of these patterns, verb followed by a particle then an object or verb followed by an object then a particle. To put on and take off (e.g. clothes) are examples of this type of verb:
Take that stained pullover off and put your tracksuit on.
Take off that stained jumper and put on your loose top and trousers.
I've put on a lot of weight since I've been driving to work.
I've put a lot of weight on since I've been using the car for work.
However, if the object is a pronoun it must be placed in front of the particle:
That sweater's stained. Take it off.
You cannot say:
That sweater's stained. Take off it.
verb + prep / adv + zero object
Note that phrasal verbs can also be intransitive where there is no object:
The plane took off five minutes early.
Two hours later it touched down in Berlin.
The meeting dragged on and on and Reginald found himself dozing off.
When he came to, he noticed that an argument had flared up.
verb + adv + prep + object
A smaller number of phrasal verbs follow the pattern of verb followed by an adverbial particle and preposition (or double preposition) and then an object. Examples include: look forward to (anticipate with pleasure), get on with (form a good relationship with OR continue to do), put up with (tolerate):
I'm looking forward to the trip. I'm looking forward to meeting Jo.
Ill get on with the ironing while you prepare supper.
He doesn't get on with his sister, but he puts up with her childishness.
verb + object + adv + prep
Occasionally phrasal verbs follow this pattern. Examples include: talk out of (dissuade from), take up on (accept), let in for (assume responsibility for):
He insisted on taking his five-year-old daughter to the football match and I couldn't talk him out of it.
Id like to take you up on your offer of employment. I'm letting myself in for a lot of work but I wouldn't want to miss out on this opportunity.