Mostrar (Spanish for “to show”) comes from the Latin root, monstrare (“to point out”), which comes from monstrum, an “omen from God; a wonder.”
From that root monstrum, we get two related English words:
We can see how the m‑n-st‑r root in the original Latin was preserved in the two English descendants, but turned into m‑st‑r in the Spanish mostrar, losing the middle ‑n-.
It’s curious how the sense of awe and wonder, of a God-given message, has been lost as monstrum — the divine omen! — turned into merely demonstrations or just showing, mostrar. Sounds like the modern world, in a nutshell.