The plural form of helmet is helmets.
Helmetful isn't in the dictionary
Bridges is the plural form of bridge.
An armet is a form of medieval full-face helmet.
"Beliefs" does not have a plural form, as it is already plural. Beliefs is the plural form of belief.
There is no plural form for the word, countries. This word itself is a plural.
"Groceries" is the plural form of "grocery."
For the animal (topi), the plural may be topior topis (like antelope).For the type of pith helmet (topi), the plural may be topis or topees.
The plural form of "meter" is "meters."
The plural form of the demonstrative pronoun 'that' is those.
The plural form for the noun sergeant is sergeants; the plural possessive form is sergeants'.
It has no plural form.
The plural form of rung is rungs.
The plural form of parenthesis is parentheses.