A simple sentence can be converted into a complex sentence by expanding a word or phrase into a subordinate clause. This clause may be a noun clause, an adjective clause or an adverb clause.

Study the following examples:

Noun clause

A word may be expanded into a noun clause.

Simple: He admitted his mistake.

Complex: He admitted that he had made a mistake. (Noun clause)

A phrase may be expanded into a noun clause.

Simple: He seems to be a fool.

Complex: It seems that he is a fool.

More examples are given below.

Simple: We heard of his success.

Complex: We heard that he had succeeded.

Simple: We know the cause of his failure.

Complex: We know why he failed.

Adverb clause

A word may be expanded into an adverb clause.

Simple: The mother hugged the child lovingly.

Complex: The mother hugged the child as if she loved her.

A phrase may be expanded into an adverb clause.

Simple: It is too hot to go out without an umbrella.

Complex: It is so hot that we cannot go out without an umbrella.

More examples are given below.

Simple: He was absent due to illness.

Complex: He was absent because he was ill.