The plural form is tablespoonfuls.
Note that tablespoonful is spelled without an 's' between 'spoon' and 'ful'.
That IS the plural - the singular is tablespoonful.
Tablespoons is already the plural for the singular tablespoon.
The plural of "she" is "they", so the plural of "she had" is "they had".
The plural of 'this' is 'these' and the plural of 'that' is 'those'.
Isthmi is plural. There are actually two plural forms. I will list them from singular to plural. Isthmus - Singular Isthmi - Plural Isthmuses - Plural
The plural of rose is roses. The plural possessive is roses'.
The plural form of can is cans.
Knights is a plural. It is the plural for knight.
applied is does not have a plural but is apply it does have a plural.
The plural is a regular plural, attics.
The plural of 'bunch' is bunches.The plural of 'ant' is ants.The plural of 'batch' is batches.The plural of 'day' is days.The plural of 'chimney' is chimneys.The plural of 'tomato' is tomatoes.The plural of 'umbrella' is umbrellas.The plural of 'donkey' is donkeys.The plural of 'sky' is skies.The plural of 'foot' is feet.The plural of 'show' is shows.
The plural for the noun loss is losses; the plural possessive is losses'.