More than the sum of the words
An idiom means something other than its individual words. Avere le mani in pasta does not mean to have your hands in the dough but to have a finger in every pie. The image stays fixed, but the verb is conjugated like any other. Most idioms belong in conversation; in academic writing they sound too casual.
Good wishes and secrets
Before an exam or a job interview, people say In bocca al lupo!, literally into the wolf's mouth, much like break a leg. The usual reply is Crepi! (may it die, meaning the wolf), not grazie. Acqua in bocca asks someone to keep something to themselves.
A
Domani ho il colloquio.
I've got the interview tomorrow.
B
In bocca al lupo!
Break a leg!
A
Crepi!
Thanks!
Sono incinta, ma acqua in bocca, eh!
I'm pregnant, but mum's the word, OK?
Feelings
Non vedere l'ora di expresses eager anticipation and takes di + infinitive. Cadere dalle nuvole means to be completely taken aback. Someone with un diavolo per capello is fuming.
Non vedo l'ora di andare in vacanza.
I can't wait to go on holiday.
Quando me l'hanno detto, sono caduta dalle nuvole.
When they told me, I was completely taken aback.
Oggi il capo ha un diavolo per capello.
The boss is in a foul mood today.
Describing people
Suo padre ha le mani in pasta in mezza città.
His father has a finger in every pie across half the city.
Having connections and influence, often with a slightly critical undertone.
La nuova collega è proprio in gamba.
The new colleague is really on the ball.
Capable, sharp, competent.
Mia nonna non ha peli sulla lingua.
My grandmother doesn't mince her words.
Money and getting things done
Il trasloco ci è costato un occhio della testa.
The move cost us an arm and a leg.
Fino a fine mese sono al verde.
I'm broke until the end of the month.
Passo dalla posta e dal medico: così prendo due piccioni con una fava.
I'll go to the post office and the doctor's, that way I kill two birds with one stone.
