IPA: //θæŋkˈfʊlər//
KK: /θæŋkˈfʊlər/
Feeling or showing gratitude; being appreciative of something or someone.
She was thankfuller for the help she received during her difficult times.
Comparative: thankfuller
Superlative: thankfullest
Thankfuller → It is formed from "thank" (from Old English *þanc*, meaning gratitude or thanks) and "-ful" (meaning full of) combined with the comparative suffix "-er" (indicating more). The word "thankfuller" means more full of gratitude or expressing greater thanks than usual.
Think of feeling 'full of gratitude' ('thank') and then imagine being 'more' ('-er') grateful than before.
No commonly confused words.