Q: After reading "The Hebrew Letters" book, how can one explain simply adding the letter nun to the letter pei to change the word for "mouth" ("peh") to "face" ("pan")? Surely the nun relates more to the soul and "fish"?
A: Peh means "mouth," and when the hei is substituted with a nun it becomes pan. One pan is a profile, and panim, the dual form of pan, is the entire face. The letter nun represents "soul." The soul shines through the face, as in the verse "Chochmat adam tair panav." The inner wisdom, which is the chaya, becomes manifest through the face. That is why the word pan actually meanspnimiut, "inside." The letter nun also means "fish" (in Aramaic). A fish is also "inside," under the water. The primary meaning ofpan is "inside."
The fundamental meaning of peh, "mouth," is poh, "here." This means "manifestation," here and now. The general state of manifestation is referred to in Kabbalah as alma d'itgalyah, "the revealed world," which corresponds to the sefirah of malchut.
Pan in Kabbalah represents the inner world, which is chochmah, which shines through the face. When an alef is added to pan it becomes anaf or anpin. It then means "a long face," arich anpin, or a "short face," ze'ir anpin. It is actually a translation of the word af. The nun disappears from the root (as is common for the letter nun) and then the word becomes af, "nose." This is the relation between the nose and mouth. In both, the sole consonant letter is the pei. When there is an alef at the beginning, followed by a nun which sometimes falls from the word, then it is anpin. There is also a nun at the end of anpin.
This is either face or nose, as the nose is the primary feature of the face.
Even in the nigleh, according to Jewish law a person is recognized by his nose. The nose protrudes from the face. A person'schochmah ("Chochmat adam tair panav") from the inner realm, pan, causes a protrusion in the face. Any protrusion is relatively a male force. The peh, "mouth", is a cavity in the face which reveals one's thoughts through speech. Thus it is called almah d'itgalya, "the revealed world". The cavity is a female image. This is the image of malchut, "the revealed world."
Wishing you much success in your further studies.