Adding the prefix of the imperfect signal letter to the nif’al נ prefix causes that נ to assimilate with the first root letter, doubling it with a dagesh: יִזָּכֵר. The clearest way to recognize a nif’al imperfect is to notice the dagesh doubling the first root letter. The vowel under the א in the first person singular form is sometimes khirik and sometimes segol. When the first root letter is י, that י transforms into a ו consonant, which is then doubled with a dagesh when the נ prefix assimilates: יִוָּלֵד.  When the first root letter cannot be doubled (א ה ח ע ר), an extra dot is added under the signal letter, changing the khirik to a tseirei: יֵעָשֶׂה, יֵאָמֵר, יֵרָאֶה