The plural of feat is feats.
The homonym of "feet" is "feat". "Feet" refers to the plural of foot (body part), while "feat" refers to a remarkable achievement or accomplishment.
The closest I can come to an antonym of feat is triviality. A feat is a notable accomplishment (such as a feat of strength) so an accomplishment that is in no way notable could be described as a triviality.
There is no plural form. Do and Do not are verbs
The plural form of him, her, or it is them. (objective pronouns)
Bridges is the plural form of bridge.
The plural form of "is" is "are."
The plural form of "I" is "we."
The plural form of "was" is "were."
"Groceries" is the plural form of "grocery."
"Beliefs" does not have a plural form, as it is already plural. Beliefs is the plural form of belief.
The plural form of mouth is mouths. The plural form of month is months. The Mounth is a range of hills in Scotland and does not have a plural form.
The plural form of the demonstrative pronoun 'that' is those.