What's the difference between an Idiom and Expression?

Idioms are a specific type of expression, so all idioms are expressions, but not all expressions are idioms. An expression is any set phrase that speakers of a language use with a specific meaning, and an idiom is a type of expression whose definition can't be derived from the usual definitions of the words that make it up (e.g., if you say an outcome is "still up in the air", that means it hasn't been decided yet, but if you had never heard that idiom before, you would have no way to derive that meaning from the phrase).

An idiom is an expression that native speakers use, but speakers of other dialects/language don't. "Idiom" refers to the uniqueness of an expression. "A cat may look at a king" is both an expression and an idiom (that is no longer relevant). Also idiomatic, though, is choosing the correct preposition after a verb or adjective. For example, "Angry about" and "angry with" are okay in formal English, but "angry at" is not. Why? Dunno, it's just an idiom.

"*In my opinion*," is an expression. It's a grouping of fixed words in a fixed order. We could also change the pronoun to, for example, '*your*', making it '*in your opinion*'. So it's a **semi-fixed expression**. If you understand the meaning of '*in*' , '*my*' and *'opinion'*, then you know the meaning of '*in my opinion*' without any extra knowledge needed. The same is true for: '*on foot*' '*give me a call*' However, there are some fixed and semi-fixed expressions where the words do have their usual meaning. The words are used in a playful, metaphorical or just unusual way. This is an idiom. For example: '*give me a hand*' It is not universal that a human hand is a symbol of assistance and help. Therefore, you would not know for sure what this expression means, just from the dictionary definition of the words. Often, idioms contain a metaphor: '*I'm snowed under at work today.*'  The metaphor is that paperwork = snow, and you're buried in an avalanche. Often, idioms have a historical origins, or come from specific jargon or fields: '*I'm struggling to make ends meet*' This supposedly comes from being too poor to afford a coat whose material  could meet in the middle of your chest (a dubious etymology). There is also a category called '*idiomatic language*' which includes things like **phrasal verbs**. Phrasal verbs are two or three-part verbs whose meaning is often hard to know from just the original verb (e.g. could you guess the meaning of 'give up' from knowing the meaning of 'give' and 'up') Idioms are not to be confused with proverbs, which are sentences that carry a piece of advice or statement about the world, often using metaphors and old-fashioned language. In summary, idioms are made of words you know, but the total meaning can be hard to guess unless you have context or experience with that idiom. Expressions are any groups of words that are often used together (like common collocations). English learners are often keen to learn idioms, because they are interesting and reveal elements of anglophone culture. They also believe that using idioms makes them sound more 'native'. However, you need to be very careful. While there are common idioms that are important and useful to learn, if you use idioms incorrectly, even a little bit, it can sound very unnatural and even result in moscommunication.  As an example, there is someone on this subreddit that posts idioms daily. They frequently get these idioms wrong and it sounds immediatly unnatural to any native speaker. This is very unhelpful to learners and it also over-emphasises the importance of idioms.