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.