According to the structure, sentences can be divided into three types

Simple

Complex -

Compound

Simple Sentence:

We know that a sentence has a subject and a predicate. But it can have different structures.

When a sentence has only one subject and one predicate, it is called a simple sentence.  

Example: 

1. I am a student.
2. I have a daily routine. 
3. I maintain my daily routine. 
4. I go to school daily.
5. I sit on the first bench. 
6. I have a friend. His name is Sumon. 
7. He shares his ideas with me. 
8. We play together. 
9. At the time he comes to our home.
10. We help each other. We are good friends.

In the above examples, every sentence is a simple sentence because each sentence has one subject, one finite verb, and one clause.

Note: The part of a sentence which contains a subject and a predicate is also called a clause.

Complex Sentence:

Example:

1. Health is wealth which is the root of all happiness.
2. Everybody knows that health is the key to success.
3. A healthy man enjoys life. 
4. Healthy man takes exercise regularly so that he can keep feet. 
5. He is so careful about his health that he does not forget to exercise. 
6. When he gets time, he plays in the open field. 
7. Though he may not have enough money, he leads a happy life.

In the above examples, every sentence is a complex sentence because each sentence has a principal clause and a subordinate clause.

A complex sentence is one type of sentence that consists of one main clause and one or more subordinate clauses.

Compound Sentence:

Example:

1. It is a small country but it has a huge population.
2. Most people live in poverty and cannot educate their children.
3. We have many students to educate but we cannot educate them. 

In the first sentence of the above passage, there are two parts: (i) Bangladesh is a small country (ii) it has a huge population. These two parts are joined by conjunction but. Moreover, each part has completed its meaning. So, the first sentence is a compound sentence. Similarly, the other sentences in the passage are compound.

When a sentence has two parts joined by a conjunction, and each part makes complete sense, it is a compound sentence.