Sentences can be categorized in terms of structure or in terms of purpose.
(A) In terms of structure
Sentences can be categorized into 3 main types:
(i) Simple sentences
(ii) Compound sentences
(iii) Complex sentences.
(i) Simple sentences
A simple sentence contains a single subject and predicate.
It describes only one thing, idea or question, and has only one verb.
It contains only an independent (main) clause.
Any independent clause can stand alone as a sentence.
It has a subject and a verb and expresses a complete thought.
Examples:
Bill reads.
Jack plays football.
Even the addition of adverbs, adjectives and prepositional phrases to a simple sentence does not change its structure.
Example:
The white dog with the black collar always barks loudly.
Even if you join several nouns with a conjunction, or several verbs with a conjunction, it remains a simple sentence.
Example:
The dog barked and growled loudly.
(ii) Compound sentences
A compound sentence consists of two or more simple sentences joined together using a co-ordinating conjunction such as and, or or but.
Example:
The sun was setting in the west and the moon was just rising.
Each clause can stand alone as a sentence.
Example:
The sun was setting in the west.
The moon was just rising.
Every clause is like a sentence with a subject and a verb.
A coordinating conjunction goes in the middle of the sentence; it is the word that joins the two clauses together.
Other Examples:
I walked to the shops, but my wife drove there.
I might watch the film, or I might visit my aunt.
My friend enjoyed the film, but she didn’t like the actor.
Note
Two simple sentences should be combined to form one compound sentence only if the ideas they express are closely related.
If the ideas are not closely related, the resulting sentence may not make sense.
Examples:
Incorrect: The car is old, and Dan likes sociology.
Correct: The car is old, but it functions superbly.
Punctuating compound sentences
When writing some compound sentences, a comma is used before the conjunction.
The comma tells the reader where to pause.
Without a comma, some compound sentences can be quite confusing.
Examples:
Confusing: Jane studied the specimen and her sister took notes. (The sentence might cause the reader to think that Jane studied both the specimen and her sister)
Better: Jane studied the specimen, and her sister took notes. (The comma makes the sentence to be clear)
Sometimes the parts of a compound sentence can be joined with a semicolon (;) rather than a comma and a conjunction.
Example:
Jane studied the specimen; her sister took notes.
Never join simple sentences with a comma alone.
A comma is not powerful enough to hold the sentences together.
Instead use a semicolon.
Example:
Incorrect: My father enjoyed the meal, he didn’t like the soup.
Correct: My father enjoyed the meal; he didn’t like the soup.
Correct: My father enjoyed the meal, but he didn’t like the soup.
(iii) Complex sentences
A complex sentence contains one independent (main) clause and one or more subordinate (dependent) clauses.
They describe more than one thing or idea and have more than one verb in them.
They are made up of more than one clause, an independent clause (that can stand by itself) and a dependent clause (which cannot stand by itself).
Example:
The picture looks flat because it is colourless.
(The picture looks flat is the independent (main) clause whereas because it is colourless is the subordinate (dependent) clause)