Complex Sentence

A complex sentence contains one independent clause and at least one subordinate clause. 
For examples,

• When students work hard, they succeed. 
• The rat ate the cheese because he was hungry.

The ways independent clause and dependent clauses are joined together may require different punctuation, i.e. 

IC+DC [ Independent clause + Dependent clause. ]

For examples,  

• I missed my favorite talk show if I left an hour later than normal. 
• We left for lunch when our group finished its report. 
• She confused her words whenever Rita tried to explain herself.

DC, IC [ Dependent clause + Comma + Independent clause. ]

For examples,

• If I left an hour later than normal, I missed my favorite talk show. 
• When our group finished its report, we left for lunch. 
• Whenever Rita tried to explain herself, she confused her words.

NOTE:
A subordinate clause or dependent clause begins with a subordinating conjunction (see DEPENDENT CLAUSE in previous lesson). Here is a list of some of the words that can be used as subordinating conjunctions: after, if, though, where, although, once, unless, wherever, as, since, until, while, because, than, when, before, that, who, whom, whenever...

Sometimes, a subordinating conjunction is a phrase rather than a single word: as if, as though, as long as, as soon as, even though, in order to, so that …