"He served as an advisor to President Lincoln, who saw Douglass as a representative for African Americans." is a complex sentence.
A complex sentence contains one independent clause and at least one or more dependent clauses.
A dependent clause always needs another clause to give a proper meaning. In the given sentence, who saw Douglass as a representative for African Americans," is a dependent clause and it will need the other clause to make sense.
Some more examples of the complex sentences are:
1. The game was all fun no matter how long it lasted.
2. Although he wanted to study abroad, the fate did not let him to.
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