A sentence, a clause or a phrase is a combination of some words; so, to understand it well, it becomes necessary to understand the function/work of each word used in them well. Without that you cannot comprehend the meaning of any text properly. Based on the work words do, we can categorize them in the following manner.

1. Naming Words (words that are names of persons, places or things):

We have already read about them. They are:

i) Nouns ii) Pronouns

2. Asserting Words (words that tell us an action or a state of the subject)

We have also read about them. They are:

VERBS

3. Describing Words (words saying quality, quantity, number, manner, etc.)

i) Adjectives ii) Adverbs

Now read the following sentences.

a) Mohan is a good boy.
b) I have two pens.
c) Ram walks slowly.
d) Ram walks very slowly.
e) Mohan is a very good boy.

[In these sentences we already know the functions of some words. They are MOHAN, IS, BOY, I, HAVE, PENS, RAM and WALKS. Of course they are nouns (Mohan, boy, pens, Ram), or pronoun (I) or verbs (is, have, walks).]

Now see what function the word GOOD is doing; obviously it’s telling us the quality of the word ‘boy’, means it is describing it. Again we see the function of the word TWO; it’s telling us the number of ‘pens’, means it is describing it. Now see the word SLOWLY; it’s saying the manner of the action walking, means it is describing the word ‘walks’. Similarly the word VERY in the forth sentence is describing the word ‘slowly’ and in the fifth sentence the word VERY is describing the word ‘good’.

We see the words GOOD, TWO, SLOWLY and VERY are describing something. Hence we can say that some words do the function of describing of something. We also see that words GOOD and TWO are describing nouns (naming words), whereas the words SLOWLY is describing a verb (walks) and the word VERY in one sentence is describing the word ‘slowly’ and in another sentence it’s describing the word ‘good’. We see that the words SLOWLY and VERY are describing words other than nouns or pronouns.

From the above we can categorize the describing words in two ways. Words those are  describing nouns or pronouns. They are called ADJECTIVES. Words those are describing words other than nouns or pronouns. They are called ADVERBS.

Now see this to understand well:

ADJECTIVES:

GOOD (describing the noun BOY)
TWO (describing the noun PENS)

ADVERBS:

SLOWLY (describing the verb WALKS)
VERY (at one place describing the adjective GOOD and at other place describing an another adverb SLOWLY)

NOTE: Articles A/AN and THE are adjectives as they always describe a noun or pronoun.

4. Relating Words (establishing relation between two things)

PREPOSITIONS

See the following sentence:

The cow is in the garden.

[Here we know the functions of every word except IN. THE is an adjective, COW is a noun and GARDEN a noun also. Then what function the word IN is doing here! If you see carefully you’ll find that IN is showing the relation between two things COW and GARDEN. Similarly, in MY HOUSE IS BEHIND THE TEMPLE, the word BEHIND is showing the relation between two things HOUSE and TEMPLE. Such words are called PREPOSITIONS. We call such words PREPOSITIONS because they take positions before nouns or pronouns.]

5. Adding Words (add two words, sentences/clauses)

CONJUNCTIONS/CONNECTORS

See the following sentences:

a) Mohan and Sohan are good friends.
b) Rahul is a good boy but Rohit is a bad boy.

[In the above sentences we have already learnt what functions the words MOHAN. SOHAN, ARE, GOOD, FRIENDS, RAHUL, IS, A, BOY, ROHIT and BAD are doing. But words AND and BUT are a new type of words for us. If we see carefully the word AND is adding two words ‘Mohan’ and ‘Sohan’. And the word BUT is adding two sentences ‘Rahul is a good boy.’ and ‘Rohit is a bad boy.’  Such words are called CONJUNCTIONS or CONNECTORS.]

6. Independent Words

INTERJECTIONS

Words that are used independently in a sentence, means having no relation with other words of that sentence are called INTERJECTIONS. These words are used to express one’s strong feelings about something.

See these sentences:

a) Ouch! That hurts!
b) Hi! How are you?
c) Oh! How wonderful!

Words in the above sentences OUCH, HI, OH are called independent words. So all of them are interjections.

So we have eight parts of speech in all namely

1. Noun
2. Pronoun
3. Verb
4. Adjective
5. Adverb
6. Preposition
7. Conjunction/Connector
8. Interjection

NOTE: A word can do different functions in different uses, means it can work as a different PART OF SPEECH at different places. To understand this see this:

a) This is my book.
b) This book is mine.

[In the first sentence the word THIS is the subject; hence a noun or pronoun. Here it’s used in place of the noun BOOK, therefore working as a pronoun. In the second sentence the subject is THIS BOOK; and the word THIS is describing the noun BOOK, hence working as an adjective.]

So a word can play a different role at different places; therefore keep in mind to see the function of the word in the group of words before deciding its parts of speech.