* Idioms are phrases or expressions that have a figurative meaning different from the literal meaning of the individual words. They often have a specific cultural context.
* Expressions are broader, encompassing any combination of words that convey meaning. This includes idioms, but also everyday phrases, single words, and even sentences.
Therefore, an expression can be used in a sentence, even if it's not an idiom.
Examples:
* Expression: "The cat sat on the mat." (This is a simple sentence)
* Idiom: "Let the cat out of the bag." (This is a figurative expression meaning to reveal a secret)
Here's why your initial statement is incorrect:
* An idiom *can* be used in a sentence, but it carries a different meaning than its individual words.
* An expression can be used in a sentence, whether it's a simple phrase, a complex sentence, or an idiom.
In short: All idioms are expressions, but not all expressions are idioms.