When do 'expressions' become an issue? Usually the teachers of English (or any other language for that matter) love to tell the students to write out new expressions from the text - especially in the case of home reading.
The dictionary definition of an "expression" is a word or a phrase (or a look on someone's face - which we are not writing down, right...)
1) The task to write out expressions from the text is not intended to be pure torture to students - it has a point too - if you write out phrasal verbs, idioms or just longer phrases from the text, you will be able to use the words better in context later on. Otherwise it would be just another word to learn. Trust me - you will write out the same word for 10 times before you really learn it... (I've done my bit of home reading too in several languages...)
2) The expressions are important because their meaning usually differs from the meaning of the words taken separately. See the entry on idioms. If you do not learn the expressions as a whole, you won't be able to recognize and use them properly in the future.
3) Repetition is another key aspect. You have to either see, hear, write or use the word in speaking for about 30 times before you have totally acquired= mastered it.
4) The most important thing is not to go through the text afterwards trying to 'put together' meaningful expressions that are actually just 3 words taken from anywhere in the text making no sense at all. (less inhibited, as tall as, have been done etc.)
5) We usually do not consider of-phrases as 100% expressions especially phrases like 'number of people', 'Streets of Rome', '2 years of my working life' and 'living-room of my home'.
So let's tackle this thing one baby step at a time...
NB! When you finally understand the way teachers see expressions in the text, the vocabulary tests will never come as a surprise to you :)
