Ibuse mobile almost exclusively, so rlthe laxk if tips is frustrating. Heres the pattern I've been able to divine:
For "ett" words:
If the word ends in a vowel, then add "n". For example, "ett äpple" is the singular and ends in a vowel, so the indefinite plural is "äpplen".
If the word ends in a consonant, then do nothing. This is why "brev" is the same for the indefinite singular and indefinite plural.