Suspect and the Spanish equivalent, sospechoso, are easy to identify and obviously the same word, both from the same Latin root, suspectus.
That’s not the interesting part. Rather, as Latin evolved into Spanish, the Latin sound ‑ct- turned into the Spanish ‑ch- sound. Think lactose/leche or octagon/ocho.
And suspect falls exactly into this pattern: the English s‑s-p-ct maps exactly to the Spanish s‑s-p-ch.
An easy way to remember the Spanish decir (to say) is through the word predict.
Predict is, literally, pre — decir — to say beforehand. Pre means “before” and the dict- maps almost exactly to the Spanish decir.
How come the decir has an extra ‑t in it to be predict? Because the Latin predecire took the grammatical form of predicatus and this form grew into English (via the French influence). A prediction in Spanish, after all, is predicho!
Thus, it is a cousin of many English words such as diction and dictionary.
Ah, one of our all-time favorite patterns and examples: leche, the common Spanish word meaning, “milk.”
Leche is a first cousin of the English lactose via a very interesting pattern: the ‑ct- to ‑ch- pattern.
Both come from the same Latin root, lactatio (literally, “suckling.”) The ‑ct- in that root remained unchanged as it entered English (because it entered via the sophisticated French) but that sound almost always turned into a ‑ch- sound as Latin evolved into Spanish. Thus the l‑ct maps to the l‑ch almost exactly.
Many other awesome words follow the same pattern: think octagon/ocho, for example. Some more coming up soon (or see the pattern page linked below).
The Spanish Decir (“to say”) comes from the Latin dictio for “word”. Its participle form is dicho — and dicho also means “saying”, in the sense of, a cliche.
Thus decir is another example of the “ct” sound in Latin turning into the “ch” sound in Spanish — and is also related to the English word… dictionary.
The Spanish mancha (“spot” or “stain”) comes from the Latin for the same, macula.
From the Latin macula, we get the English… immaculate — which literally means (knowing the negation prefix of im-) “without a stain.” So the immaculate conception truly was perfect!
How this sound changed was interesting: often Latin words with a ct- or cl- or other hard letters after a c- sound turn into a suave ch in Spanish. For a distant example, see duct and ducha, or nocturnal and noche. (The ct- is much more common than the cl‑, but they’re cousins!) Thus, we can see the m‑ch of mancha mapping to the (im-)m‑cl of immaculate.