Also called a dependent clause, a dependent statement is a phrase that is not a complete sentence on its own but can be combined with a complete sentence or independent statement to form a complex sentence. These statements always begin with dependent words that indicate a logical conclusion, frame of time, or additional detail. Dependent statements can come before or after the independent statement in a sentence.
They are not complete thoughts, so dependent statements alone cannot be complete sentences. Often, sentence fragments are simply dependent statements trying to stand on their own. Dependent statements, however, are complete sentences made dependent by a dependent word. For example, "she ran errands" is a complete sentence, but adding a word that indicates a time frame at the beginning of the phrase can make it a dependent statement: "while she ran errands." The addition of "while" indicates there is more to the thought: "While she ran errands," something else was happening.
In order to complete the thought, the dependent statement must be joined with an independent statement. For example, "he made dinner while she ran errands" joins the independent clause "he made dinner" with the dependent statement. An independent clause joined with a dependent clause is called a complex sentence.
Although a dependent statement often comes at the end of the sentence, it can appear at the beginning of the sentence. For example, "while she ran errands, he made dinner" switches the placement of the independent and dependent clauses. When a dependent clause comes at the beginning of a sentence, however, a comma must be placed after the clause.
Some dependent statements can be placed only after the independent statement. "Who" and "which" — dependent words that double as interrogative words — force the dependent statement to only occur at the end of the sentence. Generally, this restriction applies because clauses that start with "who" or "which" reference the object of a sentence, not the subject. For example, "I introduced her to Jamie, who used to work with her father." The clause "who used to work with her father" is referencing "Jamie," not "I," so it needs to be placed close to the noun it references.
Placing a "who" clause at the beginning of a sentence creates a disconnect between the dependent and independent statements, as in "who used to work with her father, I introduced her to Jamie." There is no longer any logical connection between these two thoughts. The "who" statement is read as a question, despite the lack of question mark, and the "I" statement does not answer the question.