The simplest type of sentence is called a "simple sentence." It must have a subject and a predicate, which means that it must contain a noun and a verb. It is important that it is also a complete thought. A sentence can simply be something like, "It is." It has a subject, predicate, and is a complete thought. However, that doesn’t mean that it has to be short. This is also a simple sentence, "The famous actress quickly turned away from the cameras." It can also have more than one subject and more than one predicate, like in this sentence, "My best friend and I laughed and cried through the whole movie."
To make your writing more interesting and give it a natural flow, you should have different kinds of sentences with different sentence lengths. A good way to get to that point is to learn the parts of a sentence and practice writing simple sentences. Then you can mix it up with a group of sentences like this:
I wanted to get him something really special for his birthday. I searched for weeks, but I couldn’t find anything that measured up. The day before the party, I decided to try one last store. Suddenly, there it was. It was perfect.
Time4Writing provides practice in this area. Try a sample resource from our Elementary Grammar course or browse other related courses.