The singular form of gymnastics is gymnastics. The word is a collective noun, which is a singular noun that is used to describe a group (in this case, a group of related activities).
If you do remove the 's' you get the word gymnastic, which is an adjective rather than a noun: We were impressed by her gymnastic skills.
The plural of gymnasium is gymnasiums or gymnasia.
The possessive form for the noun gymnasium is gymnasium's.Example: The gymnasium's floor was refinished over the summer.
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 "was" is "were."
The plural form of "I" is "we."
The plural form of "is" is "are."
"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.