How did Grimm’s law change Proto-Indo-European sounds in Proto-Germanic?

Grimm’s law, also known as the Germanic consonant shift, describes a systematic transformation of Proto-Indo-European (PIE) stop consonants into the Proto-Germanic sound system. This process involved three primary shifts:12

• Voiceless stops to voiceless fricatives: PIE voiceless stops (p, t, k, kʷ) shifted to Proto-Germanic voiceless fricatives (f, þ, x, xʷ). For example, the PIE t in tenuis corresponds to the th in English thin.32
• Voiced stops to voiceless stops: PIE voiced stops (b, d, g, gʷ) became Proto-Germanic voiceless stops (p, t, k, kʷ). A classic example is the PIE d in decem, which became the t in English ten.32
• Voiced aspirated stops to voiced fricatives: PIE voiced aspirated stops (bh, dh, gh, gʷh) shifted to Proto-Germanic voiced fricatives (ƀ, ð, ǥ, ǥʷ), which later developed into voiced stops in many positions (e.g., b, d, g).32

These changes were regular and coherent, establishing a foundational shift in the evolution of Germanic languages. Note that certain clusters, such as sp or st, remained unchanged, and Verner’s law later accounted for additional voicing variations based on syllable accentuation.12