Grimm's law and verner's law

Solution
Karl Verner is traditionally credited as the first scholar to note the factor governing the distribution of the two outcomes. Verner observed that the apparently unexpected voicing of Proto-Indo-European voiceless stops occurred if they were non-word-initial and if the vowel preceding them carried no stress in Proto-Indo-European. The original location of stress was often retained in Greek and early Sanskrit; in Germanic, though, stress eventually became fixed on the initial (root) syllable of all words.
The following table illustrates the sound changes according to Verner. In the bottom row, for each pair, the sound on the right represents the sound changed according to Verner's Law.

PIE	*p		*t		*k		*kʷ		*s
Grimm	*ɸ		*θ		*x		*xʷ
Verner	*ɸ	*β	*θ	*ð	*x	*ɣ	*xʷ	*ɣʷ	*s	*z

The crucial difference between *patḗr and *bʰrā́tēr was therefore one of second-syllable versus first-syllable stress (compare Sanskrit pitā́ versus bhrā́tā).
The *werþaną : *wurðun contrast is likewise explained as due to stress on the root versus stress on the inflectional suffix (leaving the first syllable unstressed). There are also other Vernerian alternations, as illustrated by modern German ziehen 'to draw, pull': Old High German zogōn 'to tug, drag' ← Proto-Germanic *teuhaną : *tugōną ← Pre-Germanic *déwk-o-nom : *duk-éh₂-yo-nom 'lead'.
The change described by Verner's Law also accounts for Proto-Germanic *z as the development of Proto-Indo-European *s in some words. Since this *z changed to *r in the Scandinavian languages and in West Germanic (German, Dutch, English, Frisian), Verner's Law resulted in alternation of *s and *r in some inflectional paradigms, known as grammatischer Wechsel. For example, the Old English verb ceosan 'choose' had the past plural form curon and the past participle (ge)coren. These three forms derived from Proto-Germanic *keusaną : *kuzun ~ *kuzanaz, which again derived from Pre-Germanic *géws-o-nom : *gus-únt ~ *gus-o-nós 'taste, try'. We would have **corn for chosen in Modern English if the consonants of choose and chose had not been morphologically levelled (compare the Dutch kiezen 'to choose' : verkoren 'chosen'). On the other hand, Vernerian *r has not been levelled out in English were ← Proto-Germanic *wēzun, related to English was. Similarly, English lose, though it has the weak form lost, also has the archaic form †lorn (now seen in the compound forlorn) (compare Dutch verliezen : verloren); in German, on the other hand, the *s has been levelled out both in war 'was' (plural waren 'were') and verlieren 'lose' (participle verloren 'lost').