Where it appears in the exam
What is it?
An idiom is an expression whose meaning does not come from adding up its words: break the ice is not about ice, and get cold feet is not about temperature. Alongside them sit the fixed discourse expressions — on the whole, as far as I'm concerned, sooner or later — which are a little more transparent than a pure idiom but just as closed: they tolerate no substitutions. Both are learned the same way: as a closed block, not as a sentence you assemble.
Why it matters in the exam
In B2 First they appear in the three Reading & Use of English parts that reward precision most. Part 1 offers four synonyms of which only one belongs to the expression. Part 2 removes its grammar word. Part 4 gives you one piece of the block and asks for the rest. And in Writing and Speaking, two well-placed expressions lift your lexical range — but only if they are exact: a half-remembered idiom draws more attention than no idiom at all.
The cognitive trap
"Your instinct: reaching for the word that best fits the meaning — "could you keep a watch on my suitcase?""
Why your brain does this: the sentence is being read for sense, and sense is fully served. Watch, eye and sight all deliver 'look after this', so the substitution produces no friction at all. Idioms are stored as whole units in a different place from ordinary vocabulary, and while you are composing meaning, that store is not the one being consulted.
Could you keep an eye on my suitcase? — the block is fixed; keep watch exists, but not here
The rule: inside an idiom, no word can be replaced by a synonym. The words have stopped behaving as words — they are syllables of a single item. Which is why the useful question in Part 1 is never which word means this? but which expression am I inside?
The repertoire Cambridge keeps reusing
"Could you keep an eye on the soup while I answer the door?"
"The tickets cost an arm and a leg."
"Relax — he was only pulling your leg."
"He got cold feet the night before the interview."
"We only eat out once in a blue moon."
"Sooner or later somebody will notice the mistake."
"Buying a better one saves money in the long run."
"For the time being we're staying with my sister."
"As far as I'm concerned, the meeting was a waste of time."
"On the whole, the course was well organised."
"To be honest, I'd rather stay in tonight."
"As a matter of fact, I've already read it."
"A couple of games helped break the ice."
"She's been a bit under the weather since the weekend."
"I think you've got the wrong end of the stick."
"Don't worry — we're all in the same boat."
The errors this part punishes
"Could you keep a watch on my bag?"
Keep watch is real English, which is what makes this dangerous: the sentence is grammatical, idiomatic-sounding and wrong for this slot.
"Could you keep an eye on my bag?"
The block is keep an eye on. Nothing inside it can be swapped.
"In the whole, the trip was a success."
One preposition changed and the expression ceases to exist. In Part 2 that preposition is exactly the piece they remove.
"On the whole, the trip was a success."
On the whole is closed: not in the whole, not on all.
"Don't worry — we're all in the same ship."
Ship is a perfectly good word for a vessel, and here the expression stops existing. That is the golden rule: inside an idiom, no word takes a synonym.
"Don't worry — we're all in the same boat."
The expression is in the same boat. Not ship, not situation.
"We only go to the cinema once in blue moons."
Idioms are not pluralised or adjusted like ordinary phrases. Feeling tempted to adjust one is a reliable sign it is not yet stored as a block.
"We only go to the cinema once in a blue moon."
The expression is invariable: always a, always blue, always singular.
"As far as I concern, the price is too high."
Very common. Concerned is a participle and needs the verb be; without it the expression lands half-built and draws more attention than not using it at all.
"As far as I'm concerned, the price is too high."
A fixed formula for introducing your own view, and a cheap win in the Part 1 essay.
Why your brain gets it wrong
The learner's short circuit
Analyse the trap by exam format
I wouldn't take him seriously — I'm fairly sure he was pulling your 1 about the promotion.
Every option is a body part, every option follows pulling your grammatically, and the context supports all four equally — nobody is physically pulling anything. The distractors are the body parts other languages choose for this same idea, which is why one of them will feel oddly right. The English block is pull somebody's leg, and nothing but memory selects it.
In Part 1 an idiom is spotted by its first half
The difficulty is not knowing the expression — it is noticing you are inside one. Read it as an ordinary sentence and you will pick whichever word best fits the sense, and in an idiom the sense decides nothing.
Strategy
When the gap sits among words that are oddly specific together (an arm and a…, keep an…, break the…), stop hunting for meaning and ask which expression begins like that. Once you recognise it, the answer is no longer chosen — it is recalled.
Buying a better machine will save you money eventually. Buying a better machine will save you money ______. (RUN)
Almost the whole block reassembles itself: run is handed to you and the long follows without effort. Then the preposition has to be produced, and there is nothing to produce it from — it carries no image and no meaning of its own, so whatever preposition is most available gets inserted. That is why the preposition is the piece of an idiom that goes missing most often.
RUN
In a closed block the preposition is not reasoned out, it is remembered: in the long run.
→ in the long run
In Part 4 the preposition is the piece most often lost
With content words the exam helps you: the key word supplies one and the context suggests another. Nothing suggests the preposition. It means nothing on its own, so no amount of reasoning recovers it — either it was memorised inside the block or it gets filled in by default.
Strategy
When you learn an idiom, underline the preposition, not the striking word. What you will forget about in the long run is not run — it is in. And that is the only part the exam looks at closely.
Idioms & fixed expressions is 1 of 82
The exam tests 82 grammar competencies across 19 families. Mastering one is the first step. Automating all 82 is passing.
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