Why don't we say usefuler? I just taught my class about comparatives and superlatives and gave them the standard rule of 1 or 2 syllables taking -er and -est and 3 or more syllables taking more or most. Their textbook told them that and told them that some words are irregular like good,bad and far. That's all the information they gave them and then one of their practice words was useful. I was afraid of it so I didn't use it as an exercise in class, but I'm looking over their homework and most of them got it right anyway: more useful.
But I can't find an explanation anywhere - why don't we say usefuler? The other words I could think of that end in -ful are bashful and fretful and I think those are both irregular like useful. Is that the rule, then? Words that end in -ful? So why didn't our textbook tell them that?