English is a rich language, with a huge vocabulary, and thousands of idioms. Often people find idioms are the easiest phrases to learn, because they are fun, but be warned - idioms are dangerous. How so?
Idioms can be a treasure trove, or a minefield. The problem with idioms is there are so many, and they rarely follow a logical pattern: What is said differs from what is actually meant.
Today I got a great example in an email that was advertising a training course for teachers. The email was trying to promote the benefits of this course, and it mentioned "The ivy on the cake". At first I was confused, then my brain clicked and I realised they meant "The icing on the cake", which refers to something that makes a good situation even better.
So, is it a problem? After all, I managed to work out what they meant. Well it was a problem for me, it was a business related email, and I deleted it, because that kind of mistake makes me think the person is not professional enough to run a teaching course that I would be interested in. Harsh? Yes, but it is reality.
This is why I teach my students to learn idioms, but to avoid using them until they are 100% confident that they understand idioms, how to use them, and most importantly when to use them.