IELTS Vocabulary: if you use an idiom...

Look at these two phrases from yesterday's lesson. Are they correct?

• save something for the rainy day
• save something for rainy days

The answer is no. Both phrases contain a mistake.
The correct form is this: save (something) for a rainy day

If you use an idiom, you need to get it exactly right. Even a small mistake in an idiomatic expression makes it sound strange and completely wrong.