Write down - write up

The addition of the adverbial particle has the function of modifying the meaning of the verb itself to create a new meaning. Sometimes the addition modifies the meaning slightly, as with these examples, although the usages are still distinct:

write = put pen to paper
write sth down = write sth on paper in order to remember it
write sth up = record sth (usually notes) in neat and complete form

• You don't have to write everything down that the lecturer says.
  
• I'm going to write up my lecture notes this evening.
  
• I'm planning to write my essay on Molière's comedies next week.

Clearly in these following examples write down and write up would be inappropriate and the non-particle verb write would be the right one to use:

• My little brother is just learning to read and write.
  
• Molière wrote his last play, The Imaginary Invalid, in 1673.

Note that with the phrasal verbs, write down and write up, the particle can be placed before or after the object:

• I wrote down your number / I wrote your number down, but now I can't find it.
  
  
• Write up your notes / Write your notes up while they're still fresh.

However, if the object is a pronoun, it must go before the particle:

I wrote it down.
Write them up.

Phrasal verbs are difficult because they don't all follow the same pattern. With some phrasal verbs, e.g. look after, come across, the particle has to come before the object as we look after people and come across people. We don't look them after or come them across:

• He had to look after his maiden aunt. There was no one else to look after her.
  
• I came across Fanny the other day. I came across her in the supermarket.
  

come up

There are a number of different meanings and uses of come up. The addition of the particle sometimes changes the meaning slightly as in these examples.

We would choose to use come up instead of come, if the person we are visiting lives further north or lives in an apartment on a higher floor. In the opposite case we would use come down. There are similar nuances with come over or come (a)round, go up, go down, etc

• Come up and see me some time.
  
• I went down to visit my daughter in Southampton last weekend.
  
• Our new neighbours are coming over / coming round for a chat this evening.

In other uses, the addition of the particle gives rise to more radical change:

come up = arise
come up = be about to happen
come up to = approach

• A number of interesting points came up at our meeting with ICI.
  
• We've got a hectic period coming up so try to work a shorter week this week.
  
• He came up to me and asked me for a light, but it's obvious I don't smoke.

buy - buy up

Buy up suggests spending large amounts of money for large amounts of something, whereas buy simply suggests paying money for something:

• I've bought him a yellow tie with pink elephants for his birthday.
  
• They bought up all the old cinemas and converted them into dance halls.

fix - fix up

What is interesting about these two verbs is that fix is used in a variety of different ways and with different meanings, whereas fix up has mainly one use and one meaning:

fix = repair
fix = set price
fix = prepare food or drink
fix = fasten so cannot move
fix up = arrange for something to happen

• Can't you get the exhaust fixed on your car? It's making too much noise.
  
• Interest rates have been fixed at 4% for the last six months.
  
• Can you fix me a sandwich? I don't have time for lunch
  
• The shelf kept falling down so I fixed it to the wall with superglue.
  
• We'd better fix up a meeting for next week. There's a lot to talk about.

phrasal verbs - learn them!

It's worth making the effort to learn phrasal verbs. They are an integral part of the English language and if you can use them appropriately, you will be seen as an accomplished user of the English language.