What is up with the silent L? The combination of o and u should be ooo as in you or ow as in loud. But for some reason, putting an L in there changes everything.
Could comes from the Old English cuðe. Remember, the ð is eth, which is a soft th sound, meaning cuðe was pronounced something like “cooth” (the oo should be like in soot and the th like the beginning of the word this). I’d also like to point out that there’s no freaking L in that word. Anyway, cuðe is the past tense of the word cunnan, to be able, the origin word for can and couth. The th changed into a d in the fourteenth century, probably because people stopped using eth as a letter. The L was then added to make the word more like should and would.
Would comes from the Old English wolde, and although I’m not sure, I think the L would have been pronounced there. It’s the past tense of the word willan, which means to will, and I’m guessing you know what that’s the origin word for. At least this one has an L in it.
Should actually has a date, having turned up in the early thirteenth century. Like the others, it comes from an Old English word, sceolde, which is the past tense of sceal, the origin word for shall. Originally, sceal implied that it was something you must do in a sense of obligation, something that we Modern English speakers don’t really use shall for (if we use it at all!), but which remains in should.
TL;DR: Could’s spelling was made to resemble would and should, where the L’s used to be pronounced.
Tony Jebson’s page on the Origins of Old English