Gabriel Bertilson writes,

Gothic is the Germanic language spoken by the ancient Goths, one of the invaders of the Roman Empire in the 3rd to 5th centuries AD. It is classified as East Germanic (the only language of that class that we know much about), as contrasted with West Germanic, the main members of which are English, Frisian, Dutch, and all the varieties of German, and North Germanic, which contains all the Scandinavian languages except for Finnish.

So why is it classified as a different language group from the other Germanic languages? Here I’ll explain two of the main differences that distinguish the sound system of Gothic from that of other Germanic languages.

The first is the absence of i-mutation in Gothic. In all other Germanic languages a vowel that in Proto-Germanic was followed by an i or j in the next syllable would change by i-mutation. In Gothic, however, this would not occur. So, for example, we have Gothic f?tjus vs. Old English fét, feet, and Gothic nazjan vs. Old English nerien, to save.

The second is that Gothic retains the original Proto-Germanic z between vowels, but in other Germanic languages this z changes to r. (This process is known as rhotacism, a change to a rho- or r-sound; a word also used for a peculiar pronunciation or excessive use of r.) So we have Gothic dius, diuzis vs. Old English deor (from which we get Modern English deer), German tier, and Icelandic dýr, animal, and Gothic huzda vs. Modern English hoard.

This change is similar to the change of intervocalic s to r that occurred in Latin honos/honor, hon?ris, honor, distinction and arbor/arb?s, arboris, tree. The genitive form originally ended in -osis, but the intervocalic s changed to r (in this and all forms besides the nominative singular). Then the nominative form changed to -or from -os to become more like the rest of the forms of the word.

Other examples of Latin rhotacism are tempus, temporis, time, and (what I just read about with astonishment) eras, you were vs. es, you are. The stem for all the forms of sum, Latin to be (except for the ones beginning with s- and f-) is es-; but the s changes to r when it happens to be between vowels. So this explains the occurrence of er- in the imperfect and future of sum, what I would have wondered about had I thought about it a bit.

And so endeth my post. A good day to you all.