Overview

A complex sentence combines an independent clause — a complete thought that can stand alone — with one or more dependent (subordinate) clauses, which cannot stand alone and function as modifiers or complements. Dependent clauses are introduced by subordinating conjunctions (for example, "because," "although," "when") or by relative pronouns (such as "who," "which," "that"). The relationship between clauses expresses time, cause, condition, purpose, contrast, or description.

Structure and common types of subordinate clauses

There are three broad categories of subordinate clauses often found in complex sentences:

  • Adverbial clauses — modify the verb in the main clause by indicating time, reason, condition, concession, or manner (e.g., "After she finished the report, she left the office.").
  • Adjective (relative) clauses — modify a noun or pronoun and typically begin with a relative pronoun (e.g., "The book that I recommended sold out.").
  • Noun clauses — act as a subject, object, or complement and often begin with words like "that," "what," or "whether" (e.g., "What you decide will be fine.").

Typical subordinating conjunctions include: although, because, since, if, when, while, unless, before, after, until. Relative pronouns include who, whom, whose, which, that.

Punctuation and word order

Placement of the dependent clause affects punctuation. When a dependent clause appears before the independent clause, writers commonly separate the two with a comma: Before the curtain rose, the actor bowed. When the dependent clause follows the independent clause, a comma is usually unnecessary unless the clause is nonrestrictive or adds parenthetical information: The actor bowed before the curtain rose. Relative clauses that add nonessential information are set off with commas: The painter, who studied in Paris, returned home.

Uses and stylistic importance

Complex sentences allow writers and speakers to show logical relationships between ideas compactly. They can express cause and effect (Because he practiced, he improved), contrast (Although it rained, we continued), condition (If you arrive early, we will wait), and time (When the bell rang, class ended). Using complex sentences judiciously improves cohesion and nuance; overuse or overly long subordinate clauses can, however, reduce clarity.

Distinctions and examples

It helps to distinguish complex sentences from related types:

  • Simple sentence: one independent clause only (She laughed.).
  • Compound sentence: two or more independent clauses joined by coordinating conjunctions such as "and" or "but" (She laughed, and he smiled.).
  • Compound-complex sentence: combines multiple independent clauses and at least one dependent clause (She laughed, and he smiled because the joke was funny.).

Example complex sentences: Although the sky darkened, the picnic continued; The candidate who won the debate gained support; Ask whether the meeting can be rescheduled. For further reading on clauses and punctuation, consult a detailed guide: Comprehensive guide to clauses and a practical punctuation resource: Grammar reference on punctuation.