A simple sentence is one that conveys or expresses one idea or thought. This type of sentence contains one independent clause that makes sense by itself. A simple sentence consists of a subject and a predicate. The predicate is that part of the sentence that modifies the subject. "The ball is blue" is an example of a simple sentence; "ball" is the subject and "blue" is the predicate.
The term "simple sentence" does not mean that the idea contained in the sentence or the language used in that sentence is necessarily simple. In fact, simple sentences do not need to be short or easy to understand, but they must contain only one independent clause. "The red-faced English teacher was late for his important staff meeting" is a simple sentence; it consists of one independent clause.
A complex sentences consists of a main or independent clause and one or more subordinate or dependent clauses, which are joined by a connecting word known as a conjunction or a connective. The main clause makes sense on its own and conveys an independent idea, like the simple sentence. The subordinate clause brings additional information and cannot stand alone. "The English teacher missed his important meeting because his bus was late" is an example of a complex sentence. "The English teacher missed his important meeting" is the main or independent clause, telling us the most important information; "because his bus was late" is the subordinate or dependent clause providing additional information. "Because" is the conjunction.
Complex sentences do not have to follow the structure of the example used above. In fact, varying the structure of complex sentences is another way of adding interest to writing. The subordinate clause can come first: "Because his bus was late, the English teacher missed his important meeting." When using this order, it is important to remember to use a comma in the correct place.