Is brev another word like barn that can be used as singular or plural depending on the context?
But, in this case, couldn't it mean "He is reading letter"?
In that case there would be an article before the word
Not any more than "He sees deer." would mean "He sees a deer." You would have to put the article for the singular. "et brev" indefinite singular or "brevet" definite singular, but "brev" by itself is indefinite plural.
That would be:
He's reading a letter = Han leser et brev
That doesn't make sense.
what if in a negative sentence : han leser ikke et brev ( what a laaazy guy cruel truth like me : < )
No, it means "He is reading letters".
could you also say "han leser brever" ?
No, the plural of "brev" is "brev".
Why is "he reads a letter" considered wrong?
because there is no use of "a" in the sentence
Because he reads several letters, not just one.
How am I supposed to know that it's more then one letter? It's out of the context isn't it
If there is only 1 letter, then you would use ett brev (one book) or a book (et brev). "He is reading book" makes no sense without multiple books.