Nouns are things. If you can stub your toe on it, it's a noun. Nobody ever stubbed his toe on an adjective. (Or her toe, sister.*)

There are, of course, plenty of things that you can't stub your toe on that are also nouns. Like Oxygen. (Unless it's liquid Oxygen and it's in a tank.)

A better test to see if it's a noun is if you can put the word "the" in front of it without sounding insane. Which you would do if you went around saying things like this:

The blue.
The happy.
The bright.

In English, you expect a noun after "the", not an adjective.

The Oxygen is in the tank.

The toe I stubbed on the Oxygen tank has turned the brightest blue.

*Pronouns have to agree in number. See the Pronoun section for what this means.

Forming plurals mostly involves adding an 's' the end of the singular:

Some are the same in the singular and the plural:

And some take 'es':

Nouns ending in 'f' or 'fe' change the plural with a 'ves':

Some plural nouns are irregular, and just have to be learned:

There are lots of irregular nouns. The website Grammarly has many more:

Lots more irregular nouns

And speaking of grammar, if you keep making mistakes with things like apostrophes and commas, there's an excellent article here that will help you out:

Dangerous Grammar Mistakes

A great read, and very entertaining!

The plural of books used to be bec. The singular used to be boc. (I've missed out the little horizontal bars over the 'e' and 'o'.)

I only want one boc.

You have lots of bec.

The plural of hand used to be hend:

Show me your left hand.

Now show me both hend.