The noun phrase 'written reminders' is the pluralform.

The singular form is a written reminder.

The possessive form of the plural noun aunts is aunts'.

Example: My aunts' names are Alice and Anne.

Yes, the noun 'herds' is the plural form of the singular noun herd.

Examples:

The farmer had a herd of sheep. (singular)

Several herds of cattle were brought to the auction. (plural)

The word 'herds' is also the third person, singular, present form of the verb to herd.

Example: He herds the cattle into the barn each evening.

The plural noun is insignia.

The singular noun is insigne.

No, the noun 'rain' is a singular, uncountable (mass) noun as a word for water drops falling from clouds; a word for percipitation.

The plural noun 'rains' is a plural uncountable (mass) noun as a word specifically for seasons or periods of rain.

The plural title for two or more men named Jones is Messrs. Jones.

There are two accepted forms for possessive singular nouns that end with an s:

Add an apostrophe (') after the existing s at the end of the word:

• Messrs. Jones'

Add an apostrophe s ('s) after the existing s at the end of the word:

• Messrs. Jones's

Examples:

The Messrs. Jones' company is called Jones & Jones.

The Messrs. Jones's company is called Jones & Jones.

Note: The plural form of the proper noun Jones is Joneses, for example:

• "The Joneses' house is on the corner."
• "The Joneses's house is on the corner."