Language & WorldHow it works
How 'pater' became 'father': Grimm's law.
Jacob Grimm, yes, the fairy-tale Grimm, described a rule in 1822: on the way into Germanic, p became f, t became th, k became h. Latin 'pater' becomes 'father', 'tres' becomes 'three', 'cornu' becomes 'horn', 'piscis' becomes 'fish'. With this rule you can spot which English and German words are true relatives of Latin words rather than later borrowings.
Useful: Latin p and English f in the same spot means ancient kinship, not borrowing.
Source: en.wikipedia.org