Nouns are one of the building blocks of the sentence. They refer to people, places, and things (including ideas). The noun could be the subject of the sentence (such as "I"), or an abstract concept such as "Happiness".
The agent is the intentful performer of an action, and is not necessarily the subject. In a passive voice sentence, the agent is not even mentioned (eg, "The equipment will be removed from the facility.")
Although the verb form used with nouns changes according to the number, sometimes it is possible to make an exception (by saying, for example, "The Rolling Stones is a popular band"). This is called synesis.
TYPES OF NOUNS
Abstract Nouns: Things which cannot be seen ("truth", "beauty", "loneliness", etc.)
Concrete Nouns: Things which can be seen and grasped easily ("rice", "bed", etc.)
Common Nouns: Any person, place or thing ("computer", "house", "book", etc.)
Compound Nouns: See a list here.
Mass Nouns (such as "weather").
Proper Nouns: Name of a specific person, place or thing (capitalised.)
In general, it seems that articles are used before CONCRETE and COMMON NOUNS.
Possessive noun: ask yourself to whom does the object, idea or quality belong?
UNCOUNTABLE NOUNS list: click here.
If an uncountable noun is the subject of a sentence, it should have a definite article.
DOUBLE NOUNS: See here.
Note the difference in meaning between "Mother Nature" and "Mother's Nature". If there is no apostrophe + s after the first noun, then the second noun has importance (Mother Nature is about nature, not mother).