Cupful is the singular form. It indicates a single full cup of something, often a food ingredient. Cupfuls is the plural, according to The Perrin-Smith Handbook of Current English, page 349. More than one full cup of something is referred to as cupfuls, as in two cupfuls of flour.
The word cupful is a singular, common noun, a word for a thing.
analysis itself is a singular form
The noun form 'cut' is singular; the plural form is 'cuts'.
The noun genius is the singular form, the plural form is geniuses.
The singular possessive form of criterion is criterion's
Cupful is singular; cupfuls is the plural.
Cupful's
The word cupful is a singular, common noun, a word for a thing.
The plural form of cupful is cupfuls.
In the word cupful, cup is the root, and ful is the suffix.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "cuffs" is cuff.
The singular form of "stratum" is "stratum." The word does not change in its singular form.
The singular form of feet is foot.
The singular form of "dice" is "die."
The singular form of "oases" is "oasis."
The singular form of "wash" is "washing."