Sorry for this noob question.
For example we have the verb nārrō, and in the dictionary both the "a" and the "o" are marked with a macron. Since "a" is before a double vowel it would be long by position even if "a" is a short vowel by nature. Therefore, even if we scan poetry and find that it is long, how is it possible that we know it definitely to be long by nature (as suggested by it being marked with a macron in the dictionary) instead of it just being the outcome of being before two consecutive consonants ? Others words like i in īnfirmus or u in fūrtim or e in lēctor also puzzles me similarly.
For example we have the verb nārrō, and in the dictionary both the "a" and the "o" are marked with a macron. Since "a" is before a double vowel it would be long by position even if "a" is a short vowel by nature. Therefore, even if we scan poetry and find that it is long, how is it possible that we know it definitely to be long by nature (as suggested by it being marked with a macron in the dictionary) instead of it just being the outcome of being before two consecutive consonants ? Others words like i in īnfirmus or u in fūrtim or e in lēctor also puzzles me similarly.