I've been learning about idioms and one that I came across was 亡羊补牢 (wang yang bu lao), which means to mend the fold after the sheep are gone. However, when I've used it in conversation, native speakers have told me it's not commonly used. Was that just because of local tendencies and idioms are commonly used elsewhere in China?
As a native speaker, I totally disagree with who thinks 亡羊补牢 is seldom used in casual situations.
It all depends on whether you use it properly.
Your friends have only one week before midterm, but he spent most of his time playing video games. Thus he is so worried that he will fail the exam. You can say 亡羊补牢为时未晚 to him to encourage him to work hard in the last week.
No matter the situation is formal or casual, as long as your friend is facing difficulties that he really cares about or other similar situation happens, you can always use 亡羊补牢 without any problem.
Why did your friends tell you 亡羊补牢 is seldom used? I think there are 2 reasons,
1, You didn't use this idiom in a proper way, but they don't want to spend time to explain how to use it properly or their English is not good enough to explain it, so they simply say it's seldom used to save troubles.
2, Some Chinese native speakers sometimes are picky with foreign speakers. They want to find errors in whatever you say to prove Chinese is the most difficult language in the world.
The best way to find out whether an idiom can be commonly used is to ask several 10-year old primary school students in China.
If most of 10-year old clearly know what this idiom means, you should be okay to use it commonly.
A child is more credible than an adult on this kind of question.
Some Chinese idioms are special, it depends on the sentences context and the say you express it.
This is the complete idioms
亡羊而补牢，未为迟也。(if is never too late if you fix the fence immediate after the first few sheep escape. )
When one say
亡羊补牢, it can either mean :
i. To comfort or remind someone that it is not too late to fix the mistake, or
ii. Somebody mess up in the first place, now they are fixing it.
iii. A hint that somebody make a mistake that must be fixed.
This is why Chinese don't simply throw out the idiom
亡羊补牢 unless you are pretty sure what you mean.
Idioms can express complex ideas in just few words. It can add literary flair to plain speech. It is used as vocabulary tool in all languages.
There are benefits in using idioms in speeches. However, it has to be in the right part of a speech, to the appropriate audience, and most importantly, be the apt idiom that can accurately describe a situation or express a feeling.
You can say idioms are not 'commonly' applied in everyday conversations. For example, it would be awkward to throw idioms around when you are discussing the menu with your waiter. It would also make you sounded pretentious to use idioms excessively.
You hear idioms mostly in formal speech, scholarly discussion, in lecture from someone of your senior or superior, a sincere conversation between friends, and among people who see themselves as cultured and highly educated.
Take 亡羊补牢 for instance, it wouldn't be out of place for a teacher who is lecturing his students or a politician who is making a speech to his peer to use this phrase in his speech. But for casual conversations, classical phrases like this are seldom utilized.
*There are many idiomatic phrases that we use in daily conversations routinely. some came from classical literature which qualify them as 'idioms' but those are in effect, 'common phrases'. For example : 有恃無恐，心知肚明