The English language has three articles: a, an, the. These articles are noun detectors. If a word can be used with one of these articles, then the word is a noun.
Every one of these words is a noun because it works with one of the articles:
the dog
a cat
an apple
the idea
an organization
a sidewalk
Here are words that do not work with any of the articles. These words are not nouns:
a crying
the sad
an over
the into
Use the articles to test a word and see if it is a noun. The articles only work with nouns.
It’s your turn. Use the articles to decide which of these words is a noun: up, fixing, window. Did you choose the third word? Good for you!