Grammar Unit: How to write Complex Sentences

Complex sentences join 1 dependent and 1 independent clause with a subordinating conjunction. 

Because I plan to eat a large plate of Mexican food on Friday after work, I will probably not lose too much weight this week. 

Notice that the yellow parts express a complete thought, and if they were taken out of the context of this sentence, they would both be a complete sentence. "I plan to eat a large plate of Mexican food on Friday after work" is made to be a dependent clause by the presence of the subordinating conjunction immediately before it. "I will probably not lose too much weight this week" is an independent clause. Notice that because the dependent clause comes first in this example, the comma is needed. 

You can also put the dependent clause after the independent clause, like this:

I will probably feel guilty about eating Mexican food unless I make up for it by exercising a little bit extra this weekend. 

Again, both yellow segments of this sentence have a subject and a verb, and if taken out of the context of this sentence, they both would be complete sentences. Because we add the subordinating clause to the front of "I make up for it...," it becomes a dependent clause. 

The only trick with complex sentences is the punctuation. If the dependent clause comes first, you need a comma. If the independent clause comes first, you do not.

More examples of complex sentences:

A. When he handed in his homework, he forgot to give the teacher the last page.
B. The teacher returned the homework after she noticed the error.
C. The students are studying because they have a test tomorrow.
D. After they finished studying, Juan and Maria went to the movies.
E. Juan and Maria went to the movies after they finished studying.

When a complex sentence begins with a subordinating conjunction (AWUBIS) such as sentences A and D, a comma is required at the end of the dependent clause. When the independent clause begins the sentence with subordinators in the middle as in sentences B, C, and E, no comma is required.

Here is a Khan Academy video about complex sentences:

Source for some of the sentences: http://eslbee.com/sentences.htm