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.
The Spanish caja (“box”) comes from the Latin capsa for the same.
This gives us a surprising connection to some English words that, on the surface, sound very different than caja:
The Latin turned into the Spanish through an interesting pattern: the -sh- sound in Latin consistently turned into the -j- sound in Spanish (at first retaining the original pronunciation, but then under the influence of Arabic, grew to the throat-clearing sound). With caja, we have a slight variation of the pattern, where the -ps- sound turned into the -j- sound. Thus, the c-ps maps exactly to c-j.
The Spanish for “chest”, pecho, sounds completely different than the English chest.
But it is related to the English word for the chest bones: the Pectoral Girdle.
The relationship is the Latin -ct- words transforming into -ch- as Latin turned into Spanish. Thus, the pect- maps to pech- exactly. The English word, on the other hand, is taken – unchanged – directly from the Latin.
Also from the same root, in Spanish, es pechuga — the common word for the common food, “chicken breast”!
The same pattern we see in noche/nocturnal, leche/lactose, etc.
The usual Spanish word for “name”, nombre, is very closely related to the English word nominal, in an interesting way. Not only does nominally mean “relating to the name”, but there is an interesting etymological pattern between the words.
Latin words with an m-n sound usually turned the m-n into an mbr sound as Latin evolved into Spanish. Thus, we see curious patterns like hominem becoming hombre, and famine and hambre being closely linked.
The same pattern applies here. The Latin nominalis turned into the Spanish nombre and the English nominal — thus the n-m-n of nominal maps exactly to the n-mbr of nombre!
The “sh” sound — often represented in writing as an “x” — transformed in all different ways to the “j” letter (and the accompanying mouth-clearing sound, influenced by Arabic) as late Latin turned into Spanish. See lots of examples: sherry/jerez, for example.
Here’s another: the common Spanish word, dejar, meaning, “to leave to the side” or “to put down” or to “put away” or to just “let go.”
Dejare comes from the Latin laxare, meaning, “to loosen”. From this same root, we get a few English words — which did not go through the x-to-j transformation Spanish did including: