A complex sentence contains one main clause and at least one dependent clause connected by a subordinating conjunction like "after", "although", or "because". There are three types of dependent clauses - adjective clauses, adverb clauses, and noun clauses. Adjective clauses modify nouns, adverb clauses modify verbs or clauses, and noun clauses serve as subjects or objects. Common connecting words introduce each type of dependent clause.

2. Definition
Complex sentences are sentences that have a main
(independent) clause and at least one dependent
clause.

3.  A main clause has a subject and verb and can stand alone.
It is a complete sentence.
EX : we’ll take a long vacation.
 A dependent clause also has a subject and verb, but
it is not complete sentence.
EX :After we have finished the project.

4. In a complex sentence, the dependent clause is connected to the
main clause by a subordinating conjunction.
The most common subordinating
conjunctions are "after," "although,"
"as," "because," "before," "how,"
"if," "once," "since," "than,"
"that," though," "till," "until,"
"when," "where," "whether,” and
while."

5. A variety of connecting words may be used to join
the clauses.
There are three basic types of dependent clauses:
Adjective Clause
Adverb Clause
Noun Clause

6. Adjective Clauses
MAIN CLAUSE
DEPENDENT CLAUSE
We are working on a project that is very difficult.
Connecting Words used in Adjective Clauses (Relative Pronouns)
Types
Relative Pronouns
Animals and Things
Which, that
People
Who, whom, that
Place
Where
Possession
Whose
Reason
Why, that
Time
when

7. Adverb Clauses
Common Connecting Words Used in Adverb Clause
(Subordinating Conjunctions)
Types
Subordinating Conjunctions
Comparison
More (less, -er ….than, as…as)
Condition
If, unless, as long as, provided that, whether or
not
Contrast
Although, even though, though, while, where
Manner
As, as if, as though
Place
Where, wherever
Purpose
So that, in order that
Reason
So…that, such…that
Result
As, because, since
Time
After, as long as, as soon as, before, once, when

8. Noun Clauses
Common Connecting Words Used in Noun Clause
(Subordinating Conjunctions)
Types
Subordinating Conjunctions
Reported Speech
that (or no conjunction)
Embedded Questions
How, if, what, when, whether,
why, and all other question
words