More than the sum of the words
An Idiom (idiom) is a fixed expression whose meaning you cannot work out from its parts. Er hat Schwein gehabt has nothing to do with pigs: it means he was lucky. Idioms make speech vivid, but they only work in exactly their fixed form.
Er hatte großes Glück.
He was very lucky.
Plain statement.
Er hat Schwein gehabt.
He got lucky.
Idiomatic, casual.
Three groups, seen from English
Same image as in English
Easy to remember, but check the details: the preposition and the article are fixed.
Damit hast du den Nagel auf den Kopf getroffen.
You've hit the nail on the head there.
Mit dem Geschenk hast du ins Schwarze getroffen.
You hit the bullseye with that present.
Das Projekt wurde auf Eis gelegt.
The project was put on ice.
Mit dem Kommentar hat er nur Öl ins Feuer gegossen.
With that comment he just added fuel to the fire.
Same meaning, different image
The trap group. The English image does not work in German.
Ich drücke dir die Daumen!
I'll keep my fingers crossed for you!
Hör auf, um den heißen Brei herumzureden.
Stop beating about the bush.
Bei dem Vortrag habe ich nur Bahnhof verstanden.
That talk was all Greek to me.
No English image at all
These you simply learn as wholes.
Sorry, ich stehe gerade auf dem Schlauch.
Sorry, I'm not getting it right now.
Mit der Frage nach seiner Frau ist sie voll ins Fettnäpfchen getreten.
She really put her foot in it by asking about his wife.
Jetzt lass mal die Kirche im Dorf, so schlimm war es nicht.
Let's not get carried away, it wasn't that bad.
Für die bessere Lage nehmen wir die höhere Miete in Kauf.
We accept the higher rent for the better location.
What can change and what cannot
The verb conjugates and moves like any verb: in the Perfekt the participle goes to the end, in a subordinate clause the verb does. The person affected goes into the right case: Das geht mir auf die Nerven. Everything else is frozen. You cannot swap a noun for a synonym, add an adjective or change the article.
Der Lärm geht mir langsam auf die Nerven.
The noise is starting to get on my nerves.
Ich weiß nicht, warum er immer um den heißen Brei herumredet.
I don't know why he always beats about the bush.
