Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
Theory is regular. It follows the -ies Conversion rule.
Use plural verbs: "The theories are ready" (not "is ready").
Yes. The plural theories is standard across all English variants.
The word theory follows the standard English pluralization rule. When forming the plural, theory becomes theories. This follows the -ies Conversion pattern, which is one of the most common plural rules in English.
Understanding why theory becomes theories helps with spelling and pronunciation. Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
When using theories in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The theories are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.