Once you can identify a basic sentence, you can join two or more sentences into complex sentences. Two or more sentences can be combined with a subordinating conjunction that explains the relationship between each idea.
For example:
Simple Sentences - He studied hard. He wanted to go to medical school. He suffered from arthritis.
Complex Sentence - He studied hard because he wanted to go to medical school as he suffered from arthritis.
Complex Sentence - Even though he suffered from arthritis, he studied hard because he wanted to go to medical school