"Marriage is a serious thing."
Translation:נישואין הם דבר רציני.
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.