父 in oracle bone form shows a hand gripping an axe or rod — the symbol of household authority in Bronze Age China. The character has barely changed since: the diagonal strokes still trace the hand and the implement. Identical across 繁體 / 新字体 / 简体.

Mandarin: fù, falling 4th tone. 父亲 (fùqīn, father — formal/written), 父母 (fùmǔ, parents), 祖父 (zǔfù, grandfather — paternal), 父子 (fùzǐ, father and son). Note that everyday spoken Mandarin uses 爸爸 (bàba) for "dad", while 父亲 carries a more formal/literary register.

Japanese is where 父 splits along a key politeness axis that English-speakers must absorb early: — 父 (chichi, kun-reading) is what you say to others about your own father — humble form. "うちの父は..." (uchi no chichi wa..., "my father..."). — お父さん (otōsan) is what you say to or about someone else's father, or how you address your own father directly. The o- prefix and -san suffix add politeness. — 父さん (tōsan) drops the o- and is more familiar. — Children may use パパ (papa) — borrowed from European languages. Sino-Japanese on-reading フ (fu) appears in compounds: 父母 (fubo, parents), 祖父 (sofu, grandfather — note the contraction).

This "humble in / polite out" pattern repeats for nearly every family-relation word in Japanese. Get the 父 / お父さん distinction right and the rest of the family vocabulary follows the same rule.

Memory aid: an X-shaped hand grip — historically clutching an axe haft, now just two crossing strokes.