i was just wondering because i see people spelling it "adds" everywhere and i seriously doubt its always a typo
it's because when you type add you hit the D key twice in the same second, once you're used to writing "adds" you tend to associate it with the phonetic sound in your head, and so you type it phonetically. Because we have muscle memory, we remember that phonetically we type "ahds" as "adds" and so we hit the D key twice when we mean to type "ad." It's an easier mistake to make than youre or dont.