It's Holly Mathers-Shuemaker :)
Both "cupful" and the plural "cupfuls" use only one L, a common short form used in measurements (teaspoonful, etc.).
No. The suffix -ful means "full" but is only a compound noun in such words as cupful or teaspoonful. In watchful, peaceful, and beautiful, it just forms an adjective.
Cupful's
Cupful is singular; cupfuls is the plural.
In the word cupful, cup is the root, and ful is the suffix.
The word cupful is a singular, common noun, a word for a thing.
Cupfuls
cupsful
cupfuls
cupfulls
No, the noun cupful is a concrete noun; a word for an amount of a substance; a word for a physical measurement.
cupful