The words "min" and "undervisar" are both pronounced with /iː/. If you're talking about how to predict vowel length, there are some general guidelines, but no rules as far as I know. Most helpfully I think is that doubled consonant letters indicate short vowels. Also, monosyllabic nouns ending in a nasal are often pronounced with short vowels despite not having a doubled consonant letter (e.g. "rum"), although the final letter is usually doubled in the definite form (e.g. rummet).