We can combine two or more sentences into a single sentence. We may do this because sentences are closely related in meaning and belong together, and because it is boring to read a series of short sentences that have a similar structure. (The preceding sentence, by the way, is a combination of 4 sentences -- can you find them?) When we talk about combining sentences, we will often use the word clause which is a group of words containing a subject and a verb. So, we will say that the sentence I know you and you know me. contains two clauses rather than saying that it contains two sentences. A sentence that contains only one clause is called a simple sentence.
PUNCTUATION: Sentences can be combined by using special punctuation marks. We know that a period [ . ] question mark [ ? ] and exclamation point [ ! ] are used at the end of a sentence, so they cannot be used for combining sentences. We also know that a comma [ , ] is not strong enough to connect 2 sentences. (If you use a comma to try to connect 2 sentences you will have a type of run-on sentence called a comma splice, which is not acceptable.)
So what kinds of puntuation marks can be used to connect sentences? There are 3 punctuation marks that are stronger than a comma but weaker than a period, question mark, or exclamation point. These are the dash [ -- ] colon [ : ] and semi-colon [ ; ].
A dash is used to add more information about some part of your sentence. It is rather informal and although you may see it when you read, it is better to avoid this punctuation in your writing.
A colon is also used to add more information and especially to give examples of something in the sentence. What follows a colon may be a clause He is a great athlete: he plays soccer, baseball and basketball. or a group of words that cannot stand by themselves He plays many sports: soccer, baseball and basketball.
A semi-colon is used to connect clauses and is the most important punctuation mark for combining sentences. It can be used alone to connect clauses He's not heavy; he's my brother. It can also be used to connect clauses together with special linking words such as however, morover, therefore which grammar books call conjunctive adverbs I think; therefore, I am. Remember, it is the semi-colon and not the conjunctive adverb that connects the clauses.
Quotation marks [ " " ] and parentheses [ ( ) ] are used to insert one sentence inside of another sentence, but this is not really sentence combination and will not be discussed here.
COORDINATION: Coordination is a way of adding sentences together. In this type of sentence (grammar books call it a compound sentence) the 2 or more sentences (clauses) which are combined are equal partners. One clause is not more important than the other grammatically. Words that connect clauses in this way are called coordinating conjunctions and the most common ones are: and, or, but, so.
SUBORDINATION: Subordination is a way of combining sentences that makes one sentence more important than the other. One sentence is under the other sentence (sub means under). Sentences that use subordination (grammar books call them complex sentences) have a main clause or independent clause and one or more subordinate clauses or dependent clauses). One or more of the sentences being combined is reduced from an independent clause to a dependent clause by adding such words as when, although, if (called subordinating conjunctions) or such words as who, what, that (called relative pronouns). Depending on its function, a subordinate clause can be classified as a:
REDUCTION: We can go one step beyond reducing one of the sentences to a subordinate clause. We can reduce it to less than clause. We can reduce it to a phrase (a group of words without both a subject and verb). A sentence may be reduced to a:
APPOSITION: In apposition, we take a word or phrase and place it in a parallel position to a noun in the sentence. An appositive is like a parenthetical statement surrounded not by parentheses but by commas.
Sara, the most serious student in the class, always did her homework. Sara was the most serious student in the class. Sara always did her homework.