Is there any pattern to tell which words change their vowel when forming the plural, like it happens with bok → böcker?

Ok so i put 'ar' word (english keyboard so i can't put the letter in there) in between pojken and laser. But it makes out that it has ruined the sentence. In a general conversation would 'ar' ruin a meaning of a sentence or would it be normal?

ok so what is the difference between bockar and bocker? like It told me to and it as a plural you had to have it as bockar when it corrected me, now it saying bocker. Halp