The Spanish verb 'demostrar' (to demonstrate) comes from the Latin 'demonstrare', which meant 'to point out' or 'demonstrate'. This Latin word was formed by combining the prefix 'de-' (meaning 'down, away') with 'monstrare' (meaning 'to show').

Going further back, 'monstrare' developed from the Latin noun 'monstrum', which referred to an 'omen' or 'supernatural occurrence'. 'Monstrum' itself was created from two parts: the verb 'monere' (meaning 'to warn or advise') and the instrumental suffix '-trum'. The evolution of meaning from 'supernatural omen' to 'showing' reflects how omens were seen as signs that showed or demonstrated divine will.

Several common Spanish words share this etymology with 'demostrar'. The verb 'mostrar' (to show) comes directly from Latin 'monstrare' without the 'de-' prefix. The noun 'muestra' (sample, display) is related, as it refers to something that shows or demonstrates. There's also 'monstruo' (monster), which connects back to the Latin 'monstrum' and its original sense of a supernatural occurrence or omen.

English speakers can connect 'demostrar' to several familiar words. Most obviously, 'demonstrate' comes from the same Latin root 'demonstrare'. The word 'monster' is also related, coming from Latin 'monstrum', and originally meant a divine omen or warning before evolving to mean a frightening creature. The word 'monitor' (to watch or check) is related too, as it comes from Latin 'monere' (to warn or advise). Even 'admonish' (to warn or reprimand) shares this ancient connection to 'monere'.