could it also be ¨the grandchild writes a letter.¨?? How can I know if singular or plural is needed here?
You can tell from the -en ending if you know the grammatical gender of the word.
Ett-words take -en in definite plural
En-words take -en in definite singular.
Since barn (and thus barnbarn too) is an ett-word, we can tell from the -en that this refers to several children.
It's a bit tricky, but that's the way it is.
It's tricky indeed. You'll get the hang of it soon enough with time and practice!
For being an ett-word that ends in a consonant, brev is actually regular. It behaves just like barn mentioned above.
Ett brev, brevet = A letter, the letter
Brev, breven = Letters, the letters.
Yes, or child’s children, or children’s children. Essentially, your grandchildren.
Coud "Barnbarnen skriver brev" also mean the grandchildren write a letter? For instance when they write a letter together.
how do you say grandchildren vs. the grandchildren in Swedish? barnbarnen vs. ?
Grandchildren = indefinite plural = barnbarn
The grandchildren = definite plural = barnbarnen
tack så mycket, so I should use barnbarnett vs. barnbarnen to tell it's definite single or pleural?
I have found that Wiktionary is a very helpful resource for seeing the different forms of words.
In this sentence, you can't - but the definite forms are different (singular brevet vs. plural breva/brevene). I would generally say in these ambiguous cases that if both singular and plural make sense, to use the same number as the subject (because it's more likely that one grandchild is writing one letter or several grandchildren are writing several letters than the other way around).
That makes a lot more sense. I probably need to review my irregular plural nouns
It's not irregular, just the regular plural-definite for neuter (ett) words.