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Unit 01
Idioms: How They Work
meaning, fixed form & register
What Is an Idiom?
An idiom is a fixed group of words whose meaning cannot be guessed from the individual words. That exam was a piece of cake has nothing to do with cake — it means the exam was very easy.
Idioms make speech natural and colourful, but they must be learned as whole units. Translating them word for word usually produces nonsense — in both directions.
Fixed Form: Don't Change the Words
Usually only the verb tense and pronouns change: She broke the ice. / He lost his temper. The other words stay the same.
| ✅ Correct | ❌ Wrong |
|---|---|
| It's raining cats and dogs. | It's raining dogs and cats. |
| a piece of cake | a slice of cake (only literal) |
| once in a blue moon | once in a red moon |
| hit the nail on the head | hit the nail on the top |
| spill the beans | spill the bean |
Register: When to Use Idioms
| Context | Idioms? | Example |
|---|---|---|
| Chatting with friends | ✅ Very natural | That exam was a piece of cake! |
| Work emails and meetings | ✅ Business idioms | Let's touch base next week. |
| Academic essays | ⚠️ Only formal fixed phrases | In the long run, the policy may succeed. |
| Formal letters | ❌ Avoid informal idioms | Not: |
How to Learn Idioms
- Learn each idiom with an example sentence, never as a single phrase on a list.
- Notice the register: most idioms are informal and best for speaking, messages and social media.
- Some idioms sound old-fashioned (e.g. raining cats and dogs). Recognise them, but prefer the common modern ones in this section.
- In exams like YDS, idioms rarely appear as direct questions, but they appear in reading passages — recognising them helps you understand the text.
Annotated Examples
Subject
Verb
Auxiliary
Object
Adjective
Adverb
Preposition
Hover over any word to see its role.
subjectThat examverbwasobjecta piece of cake.
↳ a piece of cake = very easy.
subjectMy new phoneverbcostobjectan arm and a leg.
↳ cost an arm and a leg = be very expensive.
subjectA funny storyverbhelpedobjectbreak the ice.
↳ break the ice = make people relax at the start.
Common Mistakes
It's raining dogs and cats.
→
It's raining cats and dogs.
Idioms have a fixed word order.
The test was a cake piece.
→
The test was a piece of cake.
Fixed form: a piece of cake.
We killed two birds by one stone.
→
We killed two birds with one stone.
Fixed form: with one stone.
He spilled the bean.
→
He spilled the beans.
Always the beans (plural).
Dear Sir, your prices cost an arm and a leg.
→
Dear Sir, your prices are very high.
Avoid informal idioms in formal writing.