Sentences have to be combined to avoid the monotony that would surely result if all sentences were brief and of equal length. (If you haven't already read them, see the sections on Avoiding Primer Style and Sentence Variety.) Part of the writer's task is to employ whatever music is available to him or her in language, and part of language's music lies within the rhythms of varied sentence length and structure. Even poets who write within the formal limits and sameness of an iambic pentameter beat will sometimes strike a chord against that beat and vary the structure of their clauses and sentence length, thus keeping the text alive and the reader awake. This section will explore some of the techniques we ordinary writers use to combine sentences.
A compound sentence consists of two or more independent clauses. That means that there are at least two units of thought within the sentence, either one of which can stand by itself as its own sentence. The clauses of a compound sentence are either separated by a semicolon (relatively rare) or connected by a coordinating conjunction (which is, more often than not, preceded by a comma). And the two most common coordinating conjunctions are and and but. (The others are or, for, yet, and so.) This is the simplest technique we have for combining ideas:
Notice that the and does little more than link one idea to another; the but also links, but it does more work in terms of establishing an interesting relationship between ideas. The and is part of the immediate language arsenal of children and of dreams: one thing simply comes after another and the logical relationship between the ideas is not always evident or important. The word but (and the other coordinators) is at a slightly higher level of argument.
Click here to review the rules of comma usage when you combine two independent clauses with a coordinating conjunction.
Within a sentence, ideas can be connected by compounding various sentence elements: subjects, verbs, objects or whole predicates, modifiers, etc. Notice that when two such elements of a sentence are compounded with a coordinating conjunction (as opposed to the two independent clauses of a compound sentence), the conjunction is usually adequate and no comma is required.
Subjects: When two or more subjects are doing parallel things, they can often be combined as a compounded subject.
Objects: When the subject(s) is/are acting upon two or more things in parallel, the objects can be combined.
Notice that the objects must be parallel in construction: Jefferson believed that this was true and that was true. If the objects are not parallel (Jefferson was convinced of two things: that the Missouri reached all the way to the Canadian border and wanted to begin the expedition during his term in office.) the sentence can go awry. Click here to review the principles of parallelism.
Verbs and verbals: When the subject(s) is/are doing two things at once, ideas can sometimes be combined by compounding verbs and verb forms.
Notice that there is no comma preceding the "and learned" connecting the compounded elements above.
(Notice in this second version that we don't have to repeat the "to" of the infinitive to maintain parallel form.)
Modifiers: Whenever it is appropriate, modifiers such as prepositional phrases can be compounded.
Notice that we do not need to repeat the preposition from to make the ideas successfully parallel in form.
The act of coordinating clauses simply links ideas; subordinating one clause to another establishes a more complex relationship between ideas, showing that one idea depends on another in some way: a chronological development, a cause-and-effect relationship, a conditional relationship, etc.
When we use subordination of clauses to combine ideas, the rules of punctuation are very important. It might be a good idea to review the definition of clauses at this point and the uses of the comma in setting off introductory and parenthetical elements.
The appositive is probably the most efficient technique we have for combining ideas. An appositive or appositive phrase is a renaming, a re-identification, of something earlier in the text. You can think of an appositive as a modifying clause from which the clausal machinery (usually a relative pronoun and a linking verb) has been removed. An appositive is often, but not always, a parenthetical element which requires a pair of commas to set it off from the rest of the sentence.
Notice that in the second sentence, above, Sacagawea's name is a parenthetical element (structurally, the sentence adequately identifies her as "a pregnant, fifteen-year-old Indian woman"), and thus her name is set off by commas; Charbonneau's name, however, is essential to the meaning of the sentence (otherwise, which fur-trader are we talking about?) and is not set off by a pair of commas. Click here for additional help identifying and punctuating around parenthetical elements.
A writer can integrate the idea of one sentence into a larger structure by turning that idea into a modifying phrase.
In the sentence above, the participial phrase modifies the subject of the sentence, Lewis. Phrases like this are usually set off from the rest of the sentence with a comma.
Perhaps the most elegant and most misunderstood method of combining ideas is the absolute phrase. This phrase, which is often found at the beginning of sentence, is made up of a noun (the phrase's "subject") followed, more often than not, by a participle. Other modifiers might also be part of the phrase. There is no true verb in an absolute phrase, however, and it is always treated as a parenthetical element, an introductory modifier, which is set off by a comma.
The absolute phrase might be confused with a participial phrase, and the difference between them is structurally slight but significant. The participial phrase does not contain the subject-participle relationship of the absolute phrase; it modifies the subject of the the independent clause that follows. The absolute phrase, on the other hand, is said to modify the entire clause that follows. In the first combined sentence below, for instance, the absolute phrase modifies the subject Lewis, but it also modifies the verb, telling us "under what conditions" or "in what way" or "how" he disappointed the world. The absolute phrase thus modifies the entire subsequent clause and should not be confused with a dangling participle, which must modify the subject which immediately follows.
The ideas (but not the language) about Lewis and Clark used in this section are taken from Undaunted Courage: Meriwether Lewis, Thomas Jefferson, and the Opening of the American West by Stephen E. Ambrose, Simon & Schuster, 1996. We highly recommend this book.