"One letter, several letters"
Translation:Ett brev, flera brev
man is only 'one' as the impersonal pronoun 'one' – like, "One shouldn't be too hard on oneself".
Ett brev = singular indefinite (a letter)
Brevet = singular definite (the letter)
Brev = plural indefinite (letters)
Breven = plural definite (the letters)
This happens with ett words that end with a consonant.