The Spanish hechizo (“spell”; nothing to do with the letters in words, but what a witch casts on you!) comes from the Latin facticius (“made by art”; “artificial” — indeed, that which is artificial is just something not occurring naturally but instead made by art!).
But how did artificial change to mean “spell”? Think about it this way: casting a spell goes against nature — it’s what the wicked, crazy and profoundly unnatural woman does! Think of the three weird sisters in Macbeth, and how they unnaturally stir up all the elements!
From hechizo (more specifically, from it’s Portuguese twin cognate, feitiço), we get the English fetish. How? Well, you have a fetish when the recipient casts a spell on you to become obsessed with the object of your fetish, right? Enough said!
That root facticius turned into hechizo by changing via two common patterns: the initial F in Latin tended to turn into an H as Latin turned into Spanish (compare fig and higo, or fume and humo!) and the ‑ct- tended to change to a ‑ch- (compare noche and noctural; or ocho and octagon). Thus the h‑ch of hechizo maps to the f‑sh of fetish.
The Spanish rehusar — literally, “refuse” — sounds odd to English ears: it’s the same word, but the ‑f- became an ‑h-. Huh?
This is explained via the pattern of Latin words that began with an f- tended to turn into an h- in Spanish and only in Spanish. See famine/hambre, and huir/fugitive for example.
Refuse and Rehusar follow the same pattern. Both come from the Latin refundere — from which we also get the English, refund. They are all ways of giving back.
This f‑to‑h pattern usually happens with the first letter of the word. But here it is the first letter of the second syllable — because the re- is of course the standard prefix so it didn’t effect the sound pattern change.
The Spanish “Huir” comes from the same Latin root as “fugitive”, “fugitivus”, meaning, “to flee”.
Pattern: Latin words that began with an ‘F’ tended to lose that initial ‘F’ sound and became silent (yet represented in writing with an ‘H’) as vulgar Latin turned into Spanish.
From the Latin fundus (“bottom”), we get the Spanish fondo (“background”) and hondo (“deep”) — as well as the English profound. After all, when someone says something profound, well, that’s deep.
The mapping of the Spanish f‑n-d (or h‑n-d) to the English (pro)-f-n‑d is straightforward. However, it’s curious that, in hondo, the initial F transformed from Latin into Spanish to an initial H. This is a common pattern, unique to Spanish, that we see in many Latin words as they transformed into Spanish, such as hijo and filial, refuse and rehusar, and higado and fig.
The Spanish horno, for “oven,” sounds unrelated to any English counterpart.
But it is in fact a close cousin of furnace. Both come from the Latin formus, meaning “warn”.
How did such dissimilar words end up such close cousins?
Because most Latin words that began with an f- followed by a vowel ended up evolving in Spanish (alone among the romantic languages) into an h-. Thus the h‑r-n of horno maps almost exactly to the f‑r-n of furnace. In both cases, the original ‑m- evolved into an ‑n- in the root. But that is a very common transition too, with both sounds being so similar.