There are two main indicators that a clause is subordinate:
1 The presence of a subordinating conjunction. Clauses which areintroduced by one of the subordinating conjunctions are
subordinate clauses. Here are some examples:
James left the room because he was angry.
If you need more money, just phone me.
I read a magazine while I was waiting.
However, not all subordinate clauses are introduced by a
subordinator. The subordinator that, for instance, may be
omitted:
[1] Paul knows that Amy prefers tennis.
[2] Paul knows Amy prefers tennis.
In [1] that indicates that the clause that Amy prefers tennis
is subordinate. In [2], however, there is no formal marker of
subordination, though the clause Amy prefers tennis is still
a subordinate clause. So while a subordinator always indicates a
subordinate clause, not all subordinate clauses are introduced by
a subordinator.
2 The form of the verb phrase. If the verb phrase is non-finite
then the clause in which it occurs is a subordinate clause.
We recall that the non-finite verb forms are (1) the base form
(often with to), (2) the -ed form and (3) the -ing form.
These three verb forms give their names to three subordinate clause types:
to-clauses
The road was widened to improve the traffic flow.
To receive all the channels, you may need an antenna.
A satellite must reach an altitude of 100 miles to get clear of
the atmosphere.
-ed clauses
Deprived of oxygen, plants will quickly die.
The warriors faced each other, dressed in black armour.
Designed for drafting, mechanical pencils are also useful for
sketching.
-ing clauses
Michelangelo painted lying on his back.
The teacher stood in the doorway, saying nothing.
Emily rang the doorbell, her heart pounding.
In a to-clause, to sometimes occurs as in order to or so as to:
In order to reduce heat loss, we’ve sealed the window frames.
Be punctual so as to reduce waiting time.
The form of the verb phrase, then, is a marker of subordination.
If the verb phrase is non-finite, the clause which contains it is a
subordinate clause.
