老大难
Big old trouble
【注音】lǎo dà nán
【解释】一个老问题，一个大问题，也是一个难问题。
It’s an old problem, a big problem and a difficult problem.
【例子】
例：职工们住得太远了，免不了迟到，这是一个老大难问题。

Ex.：Living too far away, the staff are often late for work. It has been a big and difficult problem for a long time.

老掉牙
Very old, obsolete
【注音】lǎo diào yá
【解释】人老了就开始掉牙齿了。
When people gets on ages, they start to lose their teeth. So it’s a sign of old age.

【例子】
例：尽管这辆自行车都老掉牙了，可是老孙还是舍不得扔掉。

Ex.：This bicycle is completely obsolete, but Lao Sun still grudges throwing it away.