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
             She is eating an apple.
             My name is John.
             What are you doing?
             Who are you?
             What a beautiful flower!

             He goes to school.

Subject and Predicate

A sentence is group of words which expresses a complete thought
   i.e. He ate an apple.

A sentence consists of two parts i.e. subject and predicate.

Sentence = Subject + Predicate

Subject

The part of sentence which performs some action in a sentence is called Subject. A subject is sometimes also defined as the part of sentence which is being spoken about in sentence (especially in sentences having a static verb). A subject has a close relationship to what is being discussed in sentence. A subject can be a noun, pronoun, noun clause or noun phrase.

Examples
           He is flying a kite.
           John is driving a car.
           She ate an apple
           I wrote him a letter.

Predicate

The part of sentence which tells about the subject is called predicate.

Examples
         He is flying a kite.
         John is driving a car.
         She ate an apple
         I wrote him a letter.

       Subject refers to the actor of a sentence. The part of a sentence about which something is told in a sentence is the subject of sentence. While the part of a sentence that tells something about subject is the predicate of sentence.

Example: She is washing her clothes.

In the above example, “she” is a subject because it performs an action of washing clothes. We can also say that “she” is a subject because something (that she is washing her clothes) is told about it in sentence. While the rest part of the sentence “is washing her clothes” is predicate of sentence because it tells something (washing her clothes) about the subject.  

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!