Determine whether a word is being used as a noun in a particular sentence by focusing on what job it is performing in the sentence. A noun's job is to refer to a person, place, thing or idea. There are many words that are both nouns and other parts of speech; "dance," for example, is both a noun and a verb. Remember to think of nouns not as a set group of words, but as functions.
Test whether a word that is hard to classify as a "person, place, or thing or idea" can be explained with simpler, common nouns. Although you can can fit most nouns into one of these four categories, abstract nouns may be trickier. Abstract nouns are nouns you cannot see, hear, smell, feel, or taste, such as "depression" or "eternity." "Depression" is a disease and "eternity" is a measure of time--both diseases and measures are easier to identify as things or ideas.
Perform a few tests on a word you suspect may be a noun. Insert "a," "an" or "the" before the word. Most nouns can be preceded by these articles or with demonstratives such as "this," "those," "these" and "that." Try making the word plural, because most nouns have distinct singular and plural forms.
Familiarize yourself with how nouns may appear. Most words ending with with -ism, -er/or, -ist, -ment, or -tion are nouns. A noun that names a specific thing is called a proper noun and will be capitalized. A possessive noun will be followed by an apostrophe and an "s" (mother's) or will end in an "s" and an apostrophe (mothers'). A compound noun is made of two words that may or may not be joined by a hyphen (sit-up, news stand).