amsterdam apophony bantu languages calvert watkins dutch language germanic strong verb gothic language greek language historical linguistics i-mutation indo-european copula indo-european languages indo-european s-mobile indo-european sound laws indo-european studies infinitive jacob grimm laryngeal theory latin linguistics manfred mayrhofer perfect aspect present tense preterite proto-indo-european accent proto-indo-european language proto-indo-european noun proto-indo-european numerals proto-indo-european particle proto-indo-european phonology proto-indo-european pronoun proto-indo-european religion proto-indo-european root proto-indo-european society proto-indo-european verb proto-semitic language reduplication sanskrit vowel vowel harmony