A complex sentence has at least one main clause and one or more subordinate clauses. The subordinate clause may be a noun clause, an adjective clause or an adverb clause.

Forming a complex sentence with a noun clause

We need justice. That should be known to everybody.

That we need justice should be known to everybody. (Here the noun clause ‘that we need justice’ is the subject of the sentence.)

The above sentence can also be written with a preparatory it.

It should be known to everybody that we need justice. (Here the noun clause ‘that we need justice’ is used in apposition to it.)

More examples are given below.

He had been promoted. I told him that.
I told him that he had been promoted. (Here the noun clause ‘that he had been promoted’ is the object of the verb told.)

They proposed to do so. I was surprised at that.
I was surprised at what they supposed to do. (Here the noun clause ‘what they proposed to do’ is the object of the preposition at.)

Forming a complex sentence with an adjective clause

These clauses are usually connected by the relative clauses who, which and that.

The thief was caught. He had stolen the bicycle.
The thief who had stolen the bicycle was caught.

This is a great opportunity. It should not be lost.
This is a great opportunity which should not be lost.

This is an old building. It was built by my grand father.
This is an old building which was built by my grand father.