Here we are trying to teach you that één means 1 and not a, which is een. Sure, using 'a' when talking may not be an issue, but it does convey a difference in meaning.
- a and een are indefinite articles.
- one and één indicate a number
Anyway, I personally think that the audio is clear enough: één is pronounced differently from een.
When accepting 'I have a brother' we are throwing away the purpose of this sentence(!), which is to teach you this difference, so I ask you to accept that we're looking for "I have one brother' as the answer instead. :)
A native Dutch speaker told me you don't need the accent marks in één. It's just that most people do it anyway. Is this true? Does it depend on your region?
It's part of standardized Dutch. The pattern is essentially as follows. To use één, the following criteria must be met:
- You're talking about the number one
- Een could be interpreted as the definite article
So essentially, it's to remove the ambiguity between the number and the definite article.
A theory about the logic behind it
Note: This theory is largely empirical.
Acute accents can be placed on the first two letters of a syllable to mark stress or emphasis:
- Dit duurt ééuwig! — This is taking forever!
- Weet je het héél zeker? — Are you really sure?
- Dat lied is écht ongelofelijk. — That song is truly incredible.
- Dat lied is echt óngelofelijk. — That song is truly incredible.
This is important in this case, because een is almost always reduced to something more like un in spoken Dutch. Eén, on the other hand, is always pronounced with a long ee. This means that, by forcing the reader to read the ee as a long ee, you can in turn force them to read it as the number.
- Ik heb een broer. — I have a brother.
- Ik heb één broer. — I have one brother.
Leaving them out
Knowing this, it makes sense that the accents are not always necessary. If from the context een can only mean the number, somebody will already read it as één:
- Een van de mensen was te laat. — One of the people was too late.
People will understand you just fine in this example. I feel like this is the reason you were told you don't need them.
wait so broer - broers is family brothers and broeder/s is metaphorical "brother" as in a church member?
"Broeder" can also mean your (family) brother, although its use is not very common.
Why is it not "I have a brother"? How can you tell if its a/an or 1 for the word Een
When it's "een" (unaccented), you have to determine whether it means a/an or 1 from context. In this sentence, "één" is used to unambiguously emphasize that the speaker has exactly one brother. The top comment thread provides much more info.