Sentences come in four basic structures
An independent clause, also called a main clause, forms a simple sentence. There are other types of sentences, each of which is determined by the number and types of clauses it contains. Each of these sentences must have at least an independent clause, which is combined with a dependent clause or dependent clauses to create other basic types of sentences. The other different types of sentences are compound sentences, complex sentences, and compound-complex sentences.
The types of sentences just referred to are as follow:
A simple sentence conveys a single idea and consists of a single independent clause or a main clause. It may have only one subject and one verb. A simple sentence can be a short one or a long one as the subject can be a compound subject and the verb too can be a verb phrase. Additionally, it may contain modifying phrases. But a simple sentence does not include a dependent clause (also called a subordinate clause) or another independent clause.
Covering with flies, the unknown thing smells strongly of rotten fish.
(Present participle phrase in bold modifying subject)
A compound sentence contains two or more independent clauses or simple sentences, but no dependent clause. It is possible for a compound sentence to have three, four or more independent clauses. But commonly, it contains only two clauses as it is a preferred way to keep sentences short.
These are the three ways a compound sentence can be formed:
Using coordinating conjunctions: and, but, for, nor, or, so, yet.
Using the semicolon or a conjunctive adverb.
Using the colon.
She sang and he listened.
(Two independent clauses forming a compound sentence are very short, a comma before the coordinating conjunction may be omitted.)
We have to leave before seven; otherwise, we will not arrive in time.
(Conjunctive adverb connecting two clauses.)
A complex sentence contains an independent clause or main clause and one or more dependent clauses, which are also called subordinate clauses. The subordinate clause has its own subject and verb, and is joined to the main clause by a subordinating conjunction, which can be as, as if, because, even if, if, unless, etc.
A subordinate clause cannot stand on its own as a sentence. It has to join a main clause to form a complete sentence as is shown here with the subordinate clauses in bold.
As she is a big bully, I stay away from her.
(If subordinate clause comes earlier in the sentence, it is separated by a comma.)
She is eating less and less because she is overweight.
(Subordinate clause follows main clause, so no comma is required.)
When she saw a rat in the kitchen, she screamed until her children rushed downstairs.
(This sentence has a main clause and two subordinate clauses.)
A compound-complex sentence contains two or more independent clauses and one or more dependent clauses. The compound-complex sentence is a combination of a compound sentence and a complex sentence. Like the compound sentence, it has at least two independent clauses, and it has at least one subordinate clause like the complex sentence.
The following show sentences containing both main clauses and subordinate clauses. The subordinate clauses are in bold.