Here are some examples

My dog is small.

I live in Stonham Aspal.

Simple sentences have 1 clause. Most simple sentences have a subject and a predicate.

E.g. My dog is small.

Subject = dog

Predicate = It is small.

A compound sentence is usually made from 2 or more simple sentences (or clauses) that are joined with a connective.

My dog is not popular with the neighbours.

It scares the postman away.

My dog is not popular with the neighbours it scares the postman away.

because

Because

But

When

Therefore

And

Although

After

While

Since

Until

Where

use the same connective

over and over again

A complex sentence is similar to a compound sentence, however when you take away the connective one of the clauses does not make sense.

E.g. The dinner was burned because she had forgotten it.

Clause 1 Connective

Clause 2

Clause 2 does not make sense on its own, without clause 1 = complex sentence.