Constants.
As far as I know, the constant pi is never used in the plural form.
Ralph seemed to have more trouble than anyone else he knew. The constant woe in his life was a constant source of pain.
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'.
"eys" like "monkeys" "keys" etc. the only exception is "monies"
applied is does not have a plural but is apply it does have a plural.
Knights is a plural. It is the plural for knight.
The plural form of can is cans.
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.