Those are the 'plural forms', "do" and "don't"; for example when you say, "they do" or "they don't".
There are some grammatical contexts, however, in which "do" and "don't" will represent the 'singular form', such as when you are talking in first-person narrative about yourself, as in "I do" or "I don't". In that particular case the context is not plural for these same words.
(As a side note, the usual 'singular forms', outside of first-person narrative, would be "does" and "doesn't", as in "she does" or "she doesn't").
Brushes is the plural form, the singular form is brush.
I dont think there is one.
The plural of igloo is igloos.
The plural form of him, her, or it is them. (objective pronouns)
Bridges is the plural form of bridge.
stupid, dont u know? Even my 5 year old sista knows that!!!
"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."
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'.