Why is “useless” spelled with 2x “s” and “useful” with just 1x”l”?

use in each has the same number of letters in it. The double “s” is in the less part. The root of the two words is the same. This, of course doesn’t explain the strangeness of the English language where rules seem to be made differently at different times for different words. Thus, less retains two s’s when combined, but full loses an “l”. If your write them out each way, the way that is incorrect doesn’t look right.

useles usefull

But that could simply be a matter of how they are used normally.

Sorry, but I’m feeling picky–“useless” actually contains 3 “s” letters.

Although, I know, you’re just making an observation about suffixes.

Because of English grammar laws that define suffixes.