Proto-Germanic is not language per se but instead a model of what theorized language which must have existed some two thousand years ago, give or take a century. This theorized language is not known from any written records or references at all, but instead has been reconstructed completely by analyzing the Germanic daughter languages. These languages (German, Dutch, Swedish, Danish, Norwegian, English, and dozens of smaller, lesser known ones like both Frisians, Luxembourgian, Faeroe, and Bavarian as well as extinct Germanic languages like Gothic and Burgundian), all shared a common ancestor and this ancestor has been designed by comparing the differences between vocabulary and phonology among the languages and deriving which forms must have existed in order to give rise to the modern forms.
     Proto-Germanic is quite an interesting theory to pursue because of the many differences between the Germanic language family and all the other Indo-European families. Germanic has undergone the most extreme sound shift (with the exception of perhaps Armenian -- after all, modern Armenian erku has evolved from older *du, and that's pretty extreme if you ask me) and the family bears the smallest amount of common Indo-European vocabulary. Linguists argue, but somewhere between 30% to 40% of all Germanic words were either bothered from a non-Indo-European source or coined within the Germanic family. Look through any etymological dictionary of any Germanic language and under a healthy number of the intrinsic Germanic words one will find the entry "ultimate source unknown." More startling, though, are the striking similarities between Germanic mythology and all other Indo-European mythologies, especially that of the Celts and, more surprisingly, the Indo-Aryan peoples. The evidence thusly supports the facts: Proto-Germanic was a language spoken by a migrant people with a strong oral tradition but no forms of writing. Writing perserves language, and the Germanic people simply did not have it while their ancestors did (the Romans were recording their history before the Germanic people were distinguishable from the Slavs). We know the Germanic people had a complex oral tradition because of the way how well preserved the Germanic myths are. This discussion will explain the evolution of Proto-Germanic, how the language itself must have functioned, and Germanic thought and religion.