An idiom is a single vocabulary item that happens to be spelled with spaces in it. You cannot swap a word inside one, you cannot reorder it, and both of the ways learners come unstuck here follow from treating it as a phrase that can be edited.
The practical consequence is an unusual recommendation: learn a great many of them to understand, and produce almost none. That is not caution for its own sake. It is that the cost of getting an idiom slightly wrong is much higher than the cost of not using one, and there is no plain sentence anywhere in English that fails because an idiom was missing from it.
Eight that are actually used, and what they mean
Idioms are worth recognising far more than producing. A learner who uses them heavily sounds like a phrasebook; one who misses them loses the point of the sentence.
- it's not rocket science, meaning it is not difficult, usually said with mild impatience.
- under the weather, mildly unwell, and the standard polite way to say so at work.
- a blessing in disguise, something that looked bad and turned out well.
- the last straw, the small final thing that ends somebody's patience after a series of larger ones.
- cost an arm and a leg, to be very expensive.
- get cold feet, to lose your nerve about something already agreed.
- on the same page, in agreement about what is happening, and extremely common at work.
- bite the bullet, to accept something unpleasant and get on with it.
Frozen means every word, in that order
It is raining cats and dogs and never dogs and cats, though nothing about the animals explains the order. It is spill the beans, not the peas. It is the last straw, not the final one, even though final is a better word for the meaning. Nothing inside is negotiable, and the reason is that the phrase was never assembled from its parts in the first place. It was learned whole by everybody who has it.
The single thing that does move is the verb, which inflects like any other: he kicked the bucket, they are spilling the beans. That is the entire flexibility available, and a learner who adjusts anything else has produced a phrase that is recognisable, wrong, and memorable in the worst way. A completely unknown phrase is heard as a mistake in English. A damaged idiom is heard as a mistake about English.
Where it stops: the dictionary has no date on it
Every idiom reference will give you the meaning. None of them will tell you whether anyone still says it, or who does. That gap is where the trouble actually lives, because the failure that costs a learner is almost never the meaning: it is producing something correct, dead, and placed.
An idiom carries an age and a region the way a coat carries a decade. A learner working from a list published in the nineties, or from subtitles of a series made in another country, acquires a set that a native speaker will understand perfectly and never say. The sentence lands, everyone follows it, and the speaker has been placed somewhere they did not choose. An out-of-date idiom is never corrected, because the listener understood it perfectly and has no reason to mention when they last said it themselves.
What a reference can and cannot settle
The idioms section of The Free Dictionary is the practical one: free, no account, and it aggregates several published idiom dictionaries into one search, so an unfamiliar phrase from a book or a film usually resolves in a few seconds. It replaced the printed dictionary of idioms, which was out of date on the day it was bound and could only be searched if you already knew the first word of the phrase.
What it will not do is the half you needed. It carries no date, no register mark you can trust, and no indication that a phrase belongs to one country or to one generation. Use it for reading, which is what it is good at, and do not use a hit there as permission to say something.
A small production set, collected rather than studied
The idioms worth saying are the ones you have personally heard said, by somebody in the room you want to be in, within the last year. That is a much shorter list than any book offers and it is the only list with a date on it. Two or three a month, collected from real speech, will beat a chapter of fifty every time, because each one arrives with the evidence about currency and register already attached.
The same problem in a faster-moving form, where the age of a phrase can be measured in a couple of years rather than a couple of decades, is what people actually say, and when not to say it. Phrases whose meaning is guessable from a repeating part are get up, put off, run into: verbs a preposition rewrites, and all of it sits under building the word count you need to say what you mean.
Since the recommendation here is comprehension rather than production, the question it raises is one about ears rather than lists: an idiom said at speed is four unstressed words that sound like one, and recognising it depends on catching a shape you were not expecting. That belongs to real speed, real accents, no subtitles.