English word for "marriage" in german. "Ehe" or "Heirat"?
I am struggling to find the difference between these two words. When are one used over the other?

Question

Answer

"Heirat" (or "Hochzeit") is the act/ceremony to start an "Ehe". Don't you distinguish between "bryllup" and "ekteskap" in Norwegian as well?

Answer

There are a few words used in German as a noun like in English, such as marriage and wedding.

Marriage, Wedding - Heirat, Hochzeit, Eheschließung and Vermählung
So in the cases above we're talking about the act of getting married.

Marriage - though can also be translated to Ehe.
Matrimony - Ehe 
Now we're talking about the act of being married over a period of time.

Hope that helps.