Anyone learning a foreign language soon becomes eager to get a few idioms (fixed expressions) under their belt. The problem is that it is sooooooooo easy to ruin the effect by getting some tiny bit wrong. The Swiss boss for whom I worked for thirteen years was fluent in English, but, my goodness, he could fracture an idiomatic expression in the tinkling of an eyeball. Among his gems, bless him, was an exhortation that we should not "sit on our laurels", a warning that it would be very bad if we had to "start again from the scratch", and a statement that an interesting thought "has just stepped into my mind".
My favourite was from before my time, in fact in the first or second year after the founding of the school. One of his teachers, a lovely man called John Bagshawe, told me how he had gone to the boss with a request for an increase in salary. The response was on the lines that the school was new, times were hard, money was tight, so "This year, Mr Bagshawe, it is not possible." Pause, and then, brightly: "But next year: ..... out of the question!"
This post is not to say that you shouldn't learn idiomatic expressions, but it is just a cautionary reminder that you need to make sure you get them exactly right, and that you know the context in which to use them.
By the way, does anyone know the origin of the expression "raining cats and dogs"?