A sentence is said to be complex when it contains both main and subordinate clauses - not because it is difficult to read or understand!

The clauses that make up a complex sentence are different from those in a compound sentence, which must be equal in semantic value; in a complex sentence, one of the clauses must be dependent on the main clause.

In the sentence above, the main clause communicates the central information in the sentence. This tells where the cat sat; but the second clause, '...because the dog had left the room' simply adds more information to the main clause.

This second clause is linked to the main clause by a different kind of conjunction, one called a subordinating conjunction, here the word 'because'. This subordinating conjunction prevents its clause from being a main clause and thus from making sufficient sense to stand alone as a sentence.

• A main clause could always be separated out from its sentence and made into a full sentence on its own; a subordinating clause, because of the subordinating conjunction it starts with, cannot.

• A subordinate clause has the function of simply adding extra information to its main clause.