Hmm, but I'd imagine you'd use "couple" the same way you'd use e.g. the word "family", that is "the family is doing something" instead of "the family are doing something".
"The young couples are dancing" - "Nuoret parit tanssivat"
"The young couple is dancing" - "Nuori pari tanssii"
Every language has it's own tricks and is hard in the beginning, but if you keep at it you'll start noticing and learning the patterns. :)
Were you wondering why it's "nuori pari tanssii" instead of "nuori pari tanssivat"? Or something else? Or just a general comment? :)