Learners love idioms because they feel like insider knowledge. They are also the fastest way to sound odd, because using one slightly wrong is far more noticeable than a plain sentence with a grammar mistake.
Understanding them is urgent; using them is not
You will meet idioms constantly in speech and film, and not knowing one can derail a whole conversation. That makes recognition genuinely important from early on. Producing them is different. A plain sentence is always acceptable; a misjudged idiom lands wrong, and you rarely find out that it did.
Register is the part that goes wrong
Most slang belongs to an age group, a region and a level of familiarity. Using a teenager’s phrase in an office, or a regional expression in the wrong city, reads as a small mistake about the situation rather than about the language. Wait until you have heard an expression used by several different people before you say it. Who says it tells you more than what it means.
Learn the ones that carry function, not colour
Worth having early: the everyday fillers and softeners that appear in every conversation — the equivalents of anyway, by the way, I mean, sort of, at least. They are technically idiomatic and they do real work. Worth leaving: vivid metaphors about animals and body parts. They are fun to collect and almost never the phrase you need.
Collect them from people, not from lists
Lists of “50 idioms natives really use” are full of expressions that are dated, regional or simply invented for the list. An idiom you heard someone say this week is a safer bet than any of them. Write down where you heard it and who said it. That context is what tells you when you can use it.
Aim to understand widely and produce narrowly. Recognise everything you hear, and say only what you have heard several real people say.