Final answer:
Rule 21 concerns English grammar and the lack of change in form for occasional count nouns between their singular and plural forms, such as 'sheep'. Some nouns ending in -s take singular verbs despite appearing plural, and rules vary for collective nouns and indefinite pronouns based on context and meaning.
Step-by-step explanation:
Rule 21 in the context refers to a grammatical rule in English which states that occasional nouns do not change form between singular and plural. This refers to certain count nouns where the singular and plural forms are the same. An example of this would be the word 'sheep', which is the same whether referring to one sheep or multiple sheep.
Additionally, there are some nouns that end in -s such as mathematics, measles, and news, which seem plural but take a singular verb as they are regarded singular in meaning. However, these rules have exceptions; for instance, if economics or politics refers to a specific situation, they may take a plural verb.
When considering subject/verb agreement in English, titles and words used as words take singular verbs, like in the title 'Da 5 Bloods centers'. Moreover, collective nouns such as 'band', can take singular or plural verbs depending on whether the group is acting as a unit or as individuals.
This extends to indefinite pronouns as well, where pronouns like 'everyone' and 'nobody' take singular verbs, while 'both' and 'several' take plural verbs. The subject-verb agreement is also influenced by verb tenses indicating the action's time frame.