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3) Complex sentences
A complex sentence includes one independent clause and one dependent
clause. An example of this is conditionals. Dependent clauses could also start
with a relative pronoun, such as who, which, or that, or they could be formed
using subordinating conjunctions, namely, because, since, after, etc.
For example

---

Because
it was late, she decided not to call him.
As
there is severe air pollution, elderlies should stay at home.
If
the government does not act now, there will be irreversible
repercussions.
The dependent clause should be followed by a comma if it
initiates the complex sentence.
I ended up reading the book for five hours 
since
it was very engaging.
A giraffe is an animal 
that
lives in Africa.
The government must make strict rules
if 
traffic needs to be
controlled.