Think of a complex sentence as a family. Although the children contribute to the family, they cannot survive on their own without the base of the family--the parents.
A complex sentence has a base of a complete sentence with a subject, verb, and words to complete the thought (the complete "couple" or "parents"). A complex sentence also adds additional information in separate phrases (the "children"). The information in the phrases depends upon the information in the complete sentence base; it cannot stand alone.
The [bracketed] phrases in the following sentences add information to the base sentence but cannot stand alone:
[If the temperature stays at about freezing], then we can join the polar bear club for a dip in the lake.
I told him that his new print on the wall looked like an interesting prehistoric drawing of a fish, [although I really just wanted to laugh].
The kids need to go to bed, [whether or not they want to], no later than 8:00 p.m.
Certain words traditionally start off the subordinate, or dependent, parts of the complex sentence:
The complex sentence is an effective way to show that one idea takes precedence over another. The idea in the complete sentence base is more important than the idea in the dependent phrase.
In the following example from one student's proposal to implement a county fire investigation team, see how he downplays certain information [in the dependent phrases] while highlighting his own ideas in the complete sentence base:
[While the effectiveness of investigations by the Sheriff's Department is not questioned], The investigations are very costly both in the amount of time personnel are taken away from other police work and the cost of paying the personnel who conduct the investigations.
[With the movement of fire investigation from the Sheriff's Department to a county fire investigation team], the investigation of fires determined to be accidental in nature could be completed by volunteer members of the team who at the most would be reimbursed mileage expenses.