This is what my English book[1] says (I've condensed the paragraphs so they're a bit shorter and straight to the point.):
Fixed expressions, also known as idioms, are often rather informal.
Never use them just to sound fluent or good at English. In a
formal situation with a person you do not know, don't say:'How do you do, Mrs Watson? Do take the weight off your feet.'
[sit down]Instead say: 'Do sit down' or 'Have a seat.'
If I understood the paragraph above correctly, does it mean we are not recommended using idioms in a formal situation? That means, we can only use idioms with friends, family or other people you have known?
[1] Vocabulary In use Upper Intermediate by McCarthy and Felicity O'Dell. Please note the authors are both born in 1947, so I don't know how recent is the book.
Best Answer
The guidance in the text is, in my view, oversimplified to the point of being incorrect.
Many idioms and fixed phrases are typically used in informal situations. For example "take a load off your feet" (or often just "take a load off") is quite informal. It should not be used in formal situations.
But idioms such as "Good things come to those who wait", "We are going to break new ground", or "They are up in arms", are neither particularly formal nor informal, and may be used on occasions all across the range from very casual to quite formal.
Some idioms and fixed phrases such as: "I am going to play devil’s advocate", "May I have the pleasure of this dance?", or "I am so pleased to meet you" are used mostly in formal writing or on formal occasions. And some fixed phrases are used only on particular formal and ceremonial occasions.
For example:
This is said by the Speaker of the US House of Representatives when the US President is about to address a joint session of Congress (a very formal occasion indeed) and at no other time.
Many ceremonies contain lots of fixed phrases. For example:
I would also disagree with the text in saying that an idiom and a fixed phrase are the same thing. I would say that they are not.
The Wikipedia article "Set phrase" begins:
while the article on "Idiom" begins:
An article from study.com reads:
Thus an idiom is a fixed metaphor or allusion, the meaning of which cannot be reliably determined from the meanings of the individual words. But a fixed phrase is a standardized expression with a fixed meaning, but that meaning can often be determined from the words of the phrase.
Idioms are a subset of fixed phrases. That is, all idioms are fixed phrases, but not all fixed phrases are idioms. But often "fixed phrase", is used to mean "a fixed phrase that is not an idiom".
I would agree that using idioms one does not well understand just to appear more fluent is often a mistake.
Therefore, when one learns an idiom, it is a good idea to learn in what situations and circumstances it is appropriate to use. This may include its level of formality, and will include its connotation as well as its denotation. It is often interesting to learn the origin or history of an idiom, but rarely is that essential to using it correctly.