These types of sentences contain one independent clause and at least one dependent clause. 

There are three types of dependent clauses: adverb, adjective, and noun.

• Complex structure with adverb clauses

Adverb clauses behave like adverbs, which means that they tell information about where, when, why, and how something happens. They start with subordinators: when, while, because, although, if, so, or that. Adverb clauses can come before or after an independent clause.

Ex.:  Although there are many causes for global warming, burning fossil fuels remains the most contributing factor to the predicament.

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• Complex structure with adjective clauses

Adjective clauses behave like adjectives. Adjective clauses describe nouns and usually start with relative pronouns: who, whom, which, whose, or, that, or with relative adverbs: where and when. They follow the noun or pronoun it describes.

Ex.: Global warming, which is more threatening than people think, is caused by burning fossil fuels.

• Complex structure with noun clauses

Noun clauses begin with a wh-question word, that, whether, and sometimes if. It behaves as the noun for the dependent clause; it could either come as a subject or an object.

Ex.: What caused global warming is burning fossil fuels.