A complex sentence is made of an independent clause and one or more dependent clauses attached to it. The independent clause can stand on its own as a sentence, while the dependent clause cannot. Examples of dependent clauses are:
because he couldn’t catch the bus.
while he waited at the train station.
since they were both short.
Dependent clauses begin with subordinating conjunctions such as: after, although, as, because, before, since, when, while, until, etc.
The dependent clauses can be attached to an independent clause to form a complex sentence:
John was late for the meeting because he couldn’t catch the bus.
He realized the train was late while he waited at the train station.
Tom and Mary couldn’t ride the rollercoaster since they were both short.
Conversely, the dependent clause can come first in the sentence, followed by the independent clause:
Because he couldn’t catch the bus, John was late for the meeting.
While he waited at the train station, he realized the train was late.
Since they were both short, Tom and Mary couldn’t ride the rollercoaster.
It should be noted that if we use the dependent clause first we should always put a comma at the end of it, before the independent clause.