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