"Marriage is a serious thing."
Translation:נישואין הם דבר רציני.
While נישואין is grammatically plural, it's not really many things, just the one. The same way you wouldn't say "מים הם חומרים שקופים" but "מים הם חומר שקוף" (water is a transparent material).
Perhaps in modern Hebrew it is different, but I believe in biblical Hebrew if a word is in plural form it is treated that way for all parts of speech.
You would. But here the word "serious" describes "thing" rather than "marriage".
Yes, and somebody's second marriage is נישואין שניים and a civil marriage is נישואין אזרחיים and so on.
Notice that the word הם is plural, and it says that the word that it modifies (נישואין) is something else. That something is the word דבר and that word is singular. So any modifier that goes with דבר is also going to be singular. Consider the following sentence in both languages:
האחים הנסון היו להקה מעצבנת - The Hanson brothers were an irritating band
The brothers are plural (in both languages) but "band" or "להקה" is singular. You see it by the form of the adjective in Hebrew or the use of the indefinite article in English.
This is similar.
What if you were saying "Marriages are serious things"? Then would it be דברים רצינים?
Sure. It changes the sentence from talking about the institution in general to be about particular marriages, maybe even all of them.
The word for marriage in Hebrew (נישואים or נישואין) is plural, so you have to use the plural copula, הם.
That's strange, as the official suggested translation uses נישואין. Anyway - חתונה is wedding, נישואין/ם is marriage.