If there is a capital letter at the beginning of a clause and a full stop at the end, you have a sentence.
The angry dog barks.
This is a simple sentence. A simple sentence consists of one clause that has a subject and a verb.
A simple sentence puts across one simple idea. For example:
There can also be several words in the verb. An example of a sentence with two words in the verb is:
It was raining.
An example of a sentence with three words in the verb is:
Sean has been running.
In each of the examples there is just one main verb and one idea. This is the simplest type of sentence and is usually very easy to understand.