A simple sentence is one that contains a subject and a verb. The verb of course being the doing word and the subject is the doer of the verb. Simple sentences can also have an object, which is the one that the subject does the verb (action) to. For example, the sentence, "Billy walks across the road." In this simple sentence Billy (subject) is the one who walks (verb), and he walks across the road (object). A simple sentence therefore makes one statement by using one verb and one subject.
Sometimes there is a need to say something more in a statement, and this requires the addition of extra material in the sentence. English grammar provides a way to join clauses of information into compound sentences by the use of commas and conjunctions. The simple sentences that are combined into a compound sentence and are called independent clauses, which means that they could stand alone as complete sentences if they were split apart again. Therefore, the sentence "Billy walked across the road, and he inspected the new car" contains two verbs and two subjects, makes two statements and is called a compound sentence.
A complex sentence contains an independent clause and one or more dependent clauses. A dependent clause is one that cannot stand alone as a sentence and needs to be joined to another sentence to make sense. A complex sentence still has two verbs and subjects, but only one of its clauses is independent. An example of a complex sentence is: "When Billy walked across the road, he inspected the new car." The first clause does not make sense by itself and needs the second clause to complete the thought.
The fourth type of sentence is the compound-complex sentence, which combines aspects of both types of sentences seen thus far. A compound-complex sentence contains two independent clauses and one or more dependent clauses. An example of a compound-complex sentence is: "When Billy walked across the road, he inspected the new car, and he drove it around the block." The first clause is a dependent clause as it has a dependent word "when," but the next two are independent clauses, which could be complete sentences on their own.
In most cases English sentences are arranged in subject-verb-object order and can be said to be an SVO language. These are the four types of sentences that are encountered in the English language. The types can easily be discerned by first looking for the verbs and the subjects to determine whether the clauses are dependent or independent. By numbering the clauses it can be determined what type of sentence -- simple, compound, complex or compound-complex -- is being used.