Rule of thumb: In vowel verbs, the basic-ending "-ru" is completely replaced by the new ending (e.g. "iru" / "eru" → "inai" / "enai"), while in consonant verbs, the "-r" sound is kept and followed by another vowel, to which the new ending is added (e.g. "iru" / "eru" → "iranai" / "eranai"). However, in the "-te/-ta" forms, consonant verbs drop the "-r" sound and replace it by "-tte/-tta" (e.g. "itte," "itta" / "ette," "etta"), while vowel verbs have "-te/-ta" (e.g. "ite," "ita" / "ete," "eta").