"Stimu" is not a word in English. "Stimulus" is singular. "Stimuli" is plural.
There are many recently invented "words" that are formed using stimu- as a root: stimu-liar, for example. These have their basis in the US government's stimulus programs. Many such words enter the language and pass from use as society ebbs and flows. When the reach the status of long term wide-spread use, they can become accepted words.
The Latin root stimu translates to goad
No one is a pronoun and is singular.
Atrium is singular - the plural is atria.
The word pictures is plural for the singular noun picture.
singular
The Latin root stimu translates to goad
The word stimuli is Latin in origin and the root of the word is stimu.
"Has" is singular, e.g. He has, she has. "Have" is plural, e.g. They have, we have. The exception is "I" - e.g. I have.
The word singular is an adjective. Adjectives do not have singular or plural forms; adjectives have comparative forms: positive: singular comparative: more singular superlative: most singular
practitioner is singular (plural practitioners)sofa is singular (plural sofas)satellite is singular (plural satellites)clips is plural (singular clip)dentist is singular (plural dentists)dollars is plural (singular dollar)article is singular (plural articles)magazines is plural (singular magazine)laminator is singular (laminators is plural)radios is plural (singular radio)
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
singular
I AM, You (singular) ARE, He/She/It IS . . . . . Plural We/You/They ARE
Fruit is singular. Fruits is plural.
No one is a pronoun and is singular.
Atrium is singular - the plural is atria.
Autumn is singular. Autumns is plural.