A complex sentence is composed of two simple sentences, each with a subject and verb, except one of the sentences has a subordinating conjunction or relative pronoun added, turning it into a dependent (or subordinate) clause. This structure allows the writer to show a sophisticated relationship between ideas, using the dependent clause to add information about the independent (or main) clause. The dependent clause can come before or after the independent clause.
Dependent clause before independent clause
When the dependent clause begins the sentence, it acts as introductory material, and a comma comes before the independent clause.
When everyone had boarded, the ship began its three-hour tour.
As soon as the castaways waded ashore, the Professor began building a radio.
Even if the Howells are rescued, they will never leave the desert isle.
(NOTE: Subjects are bolded and verbs italicized throughout this page)
Dependent clause after independent clause
Don’t use a comma if the dependent clause ends your sentence except for cases of extreme contrast.
The skipper battened down the hatches in case the weather started getting rough.
The tiny ship would have been lost if the crew had not been courageous.
The seven castaways longed for rescue, although they were living in paradise.
These subordinate conjunctions and relative pronouns frequently begin dependent clauses:
as long as
as soon as
by the time
in order that
in the event
the first time
whether or not
Relative clauses are subordinate (dependent) clauses that modify a word, phrase, or idea in the main (independent) clause. Relative clauses start with a relative pronoun: who/whom, whoever/whomever, whose, which (in which, through which, about which, for which) that, and sometimes where, when, what, why.
Maryanne wore the dress that she borrowed from Ginger.
The castaways built small huts where they spent much of their time.*
An interesting quirk of this type of dependent clause is that sometimes the relative pronoun takes the place of the subject. Let's take two simple sentences:
The Professor was once a Boy Scout.
The Professor built a radio out of coconuts.
See how "The Professor" seems a little repetitive? Let's avoid that by turning the first sentence into a relative clause with the relative pronoun replacing the subject:
(who) was once a Boy Scout
and add that relative clause to the second sentence to tell us a little more about The Professor:
The Professor, who was once a Boy Scout, built a radio out of coconuts.
Notice that the relative clause is inserted as close to the word it modifies as possible. Also note the punctuation. Commas are used at each end, as if the clause is being taped in place.
In this case, we perceive that the most important information being conveyed is that The Professor built a radio out of coconuts; however, if the most important information is that The Professor was once a Boy Scout, we could turn the second sentence into a relative clause instead:
The Professor, who built a radio out of coconuts, was once a Boy Scout.
A relative clause of this type does not necessarily get inserted into the middle of the main sentence. Let's construct another complex sentence with a relative clause using these two simple sentences:
The Skipper yelled at Gilligan.
Gilligan had climbed a palm tree.
Turn one sentence into a relative clause with the relative clause replacing the subject:
(who) had climbed a palm tree
and add it to the other sentence to tell us a little more about Gilligan:
The Skipper yelled at Gilligan, who had climbed a palm tree.*
Here, the relative clause makes best sense at the end of the sentence because that's where the word it modifies is located.
*See our comma handout for rules regarding restrictive vs. nonrestrictive clauses.