"The boy reads a letter."
Translation:Pojken läser ett brev.
Well, as you posted this five months ago, I imagine you have figured this out by now. For anyone else currently going through this lesson (like me!), the answer is "no." As I understand it so far, the only way to know is to memorize which words go with "en" and which ones go with "ett."
It's ett brev ('a letter' as in 'a love letter') but en bokstav ('a letter' as in A, B, C). If you put the wrong article, the computer thinks the easiest way to fix it is to keep the article and switch the noun to one that goes with that article. Not very clever, but that's how it works at the moment, I'm afraid.
One thing I've found that really helps me with this is to try to learn the word in the definite form, rather than in the indefinite form. For example: flicka (girl) - I learn it as 'flickan' (the girl) ... or ... hus (house) - I learn it as 'huset' (the house)
That helps me to get the gender and endings thoroughly attached to the word.
I realised (because I took a snapshot of it on my phone) that I had spelled Pojken wrong. It had a typo l in it and I think that made it correct to Killen. So if you got this error maybe check you spelled it right?
I believe* killen is akin to the English word guy.
*Not a native speaker just what I saw when checking it up online.
That's correct, kille is a little more colloquial and can be older + can mean 'boyfriend' on its own. You're right that the machine isn't very good at showing you what's really wrong with your answer – if you put en instead of ett, it will try to replace the noun instead of the article for instance, that kind of thing. The computer doesn't think like a human yet :D
Yes, you've understood wrong. en and ett have nothing to do with "a" and "an".
In English, you use "an" if the next sound is a vowel sound, and "a" otherwise.
In Swedish, it depends on the gender of the word, which can be either en or ett. Just like German ein/eine, Spanish un/una, and so on.