一去不复返 (yí qù bú fù fǎn) — (set phrase) gone never to return; once gone, never to be seen again; (figurative) irretrievably lost (of youth, opportunities, an era, etc.)

An abstract idea shown by a mark or a diagram rather than by a picture of an object.

一 is drawn as one horizontal stroke, marking the number one.

Two or more meaningful parts placed together so that their meanings combine into a new one.

去 pairs 大, a man, with 口, a mouth or object. The evidence reads this as a man leaving a place, a man opening his mouth, or a cover over an object.

A drawing of the thing itself, worn down over three thousand years into a few strokes.

The picture in 不 originally showed a flower’s calyx; the “no” sense came later from a related character, not from this picture.

Most characters — around four in five — are built this way: one part hints at the meaning, the other at the pronunciation.

This origin belongs to the traditional form 復; 复 is its simplified writing.

The simplified 复 stands for more than one traditional character. In most words (复习, 恢复) it is 復; in others (复印, 复杂) (答复, 反复) it is 複 or 覆.

The traditional form 復 combines 彳, a walking part, with 复, a sound part, to show walking back and forth.

Most characters — around four in five — are built this way: one part hints at the meaning, the other at the pronunciation.

返 pairs 辶 (movement) with 反 (sound, with the sense of returning); the whole character originally meant coming back along the path one came from.