When it comes to expressions and idioms, you learn them and use them exactly as they are; there is no room for improvement, grammar corrections, free styling or synonyms.
You can’t say things like “Nobody is happy in the house if the mom is not happy”; nope, not an option.
It’s raining cats and dogs – never “dogs and cats”, and most definitely, never any other animals.
There are different degrees of idiomaticity: some expressions have meanings that are easy to guess (example: “to bite more than you can chew”) and others have to be learned because at first they appear to be a total mystery (example: “to fall off the wagon” or “to put your money where your mouth is”). Obviously, in the first case the meaning is that someone has taken more responsibilities or work than they can handle, or gotten into a situation which is more demanding than they thought it would be. Anyone can figure it out without much effort. However, if you fall off the wagon, you fail at something you decided to change or pursue, like a diet or a fight with an addiction. You were sober but relapsed into drinking? Yep, you fell off the wagon. You hopped on the wagon with good people and support and really good intentions – but lost your nerve or something happened and now you’re back into the problem, off the wagon. The idiom about the money and the mouth is my favorite because I totally misunderstood it the first time I run across it and thought it was supposed to mean that money doesn’t buy happiness and you can stick it wherever. In reality, however, you put your money where your mouth is when you keep your promise! Crazy, I know!
Once you get into the territory of idioms, you have reached an intermediate level of learning a language – English, French, Spanish or any other. You vocabulary is rich enough and you are able to use it comfortably in a more eloquent way. The trick here is to remember and use the idioms mot-à-mot, as the French say, word-for-word. No improv allowed. If you say that you dropped off the wagon… you said nothing. If you announce that you have more in your mouth than you are able to chew, it doesn’t cut it. Sometimes it’s funny, but most of the time it’s just plain awkward. Don’t. No, seriously, don’t. You were doing so well showing off all your knowledge; don’t ruin everything by declaring you have put your “blood, tears and sweat” into that project – it’s “blood, sweat and tears” in that exact order. If you need a drink, you can’t say that in some time zone it’s probably 5 pm and socially acceptable; “it’s 5 o’clock somewhere”, no variations. Well, except in French, where cultural differences manifest themselves in a very amusing way, by correcting the acceptable time for a drink to noon: “midi” is a good time to have a glass of wine with lunch in more liberal cultures.Idioms are fun. Research their origins, find cartoons with their meanings, write a few sentences using them in your own way… They are the salt of the earth, the Holy Grail of language learning, the cherry on top… you get the idea!