We have learnt that dependent clauses cannot exist on their own. Now because they are just a part of another clause, usually the principal clause, they are subordinate in importance (sub meaning under, lower in the order)...so dependent clauses are often called subordinate clauses.
In summary, if we add a subordinate clause to a principal clause, we have turned a simple sentence into a complex sentence (because it now has many clauses, many verbs).
From the previous table, we can see that there are three types of subordinate clauses:
1. Adjectival clauses, which qualify nouns and pronouns (and are introduced by relative pronouns who, whom, whoever, whomever, that, which), e.g.:
- who had a lot of experience in robotics (qualifies engineer)
- that could make simple decisions (qualifies robot)
2. Adverbial clauses, which modify verbs (and are introduced by conjunctions such as because, if, when, whenever, although, unless...), e.g.:
- because some jobs in the factory were dangerous for humans (modifies built)
3. Noun clauses, which can act anywhere that a noun can, e.g.:
- Whoever built the robot was a genius. (Noun clause as subject of was.)
- The engineer built what is known as a humanoid robot. (Noun clause as object of built.)
- What I noticed was that the robot could make some simple decisions. (Noun clauses as both subject and complement of the verb was.)
One final point should be made about dependent clauses. It is possible to subordinate a dependent clause to another dependent clause, which creates a really complex sentence: The mountaineers went to bed early because the next day they would begin a climb that would test the limits of their physical endurance. (The final adjectival clause is attached to an adverbial clause that is attached to the principal clause!)