A complex sentence contains one independent clause and at least one dependent clause.
E.g. I passed the exam. ➜ This is an independent clause.
E.g. because I studied very hard ➜ This is a dependent clause.
There are two ways in which we can combine the IC & DC.
➜ When the independent clause comes first; then, a comma is not required to join the independent clause and the dependent clause, so the sentence is: I passed the exam because I studied very hard.
➜ When the dependent clause comes first; then, a comma is required to join the dependent clause with the independent clause, so the sentence is: Because I studied very hard, I passed the exam.