Sentence
A group of words which expresses a complete idea or thought is called a sentence.

Example
He bought a car.

The above group of word expresses a complete idea. Such combination of words is called a sentence.

Now, look at another group of words “book a read him”. This group of words does not make a complete sense so it cannot be called a sentence.
A sentence is a combination of words including a subject and a verb which express a complete idea. A sentence begins with a capital letter and ends with a period, question mark or exclamation mark.
Examples
He goes to school.
She is eating an apple.
My name is John.
What are you doing?
Who are you?
What a beautiful flower!
Kinds of Sentence
Sentence
A group of word that expresses a complete idea or thought is called a sentence.
Example.
He bought a book.
The above group of words “He bought a book” expresses a complete though hence it is a sentence. A sentence begins with capital letter and ends with a period, question mark or exclamation mark.

Kinds of sentences

There are four kinds of sentences.

Assertive or Declarative Sentence
A sentence that makes a statement or assertion is called an assertive or declarative sentence. Assertive sentence ends with a period.
Examples
He goes to school
He likes to play chess.
They are singing a song.

Interrogative Sentence
A sentence that asks a question is called an interrogative sentence. Interrogative sentence ends with question mark.
Examples.
Where are you going?
Do you use your laptop?

Imperative Sentence
A sentence that expresses a request, command or advice is called an imperative sentence.
Examples
Open the door. (an order)
Please help me. (a request)

Exclamatory Sentence
A sentence that expresses strong feelings or emotions is called an exclamatory sentence. These sentences express surprise, joy, sorrow, appreciation, love excitement, frustration, anger etc. An exclamatory sentence ends with exclamation mark.
Examples
What a beautiful flower it is!
How nicely she is singing!
That is fantastic!
Hurrah! We won the match!