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

Study the following examples

By expanding a noun into a noun clause

A noun may be expanded into a noun clause.

He admitted his mistake.
He admitted that he had made a mistake. (Here the noun mistake is expanded into the noun clause ‘that he had made a mistake’.)

We heard of his death. (Simple)
We heard that he had died. (Complex)

A noun phrase can also be expanded into a noun clause.

He seems to be a fool. (Simple)
It seems that he is a fool. (Complex. Here we expand the phrase ‘a fool’ into the clause ‘that he is a fool’.)

I would like to know the secret of your success. (Simple)
I would like to know how you succeed. (Complex)

By expanding a word or phrase into an adverb clause

A word or phrase can be expanded into an adverb clause.

It is too hot to go out without an umbrella. (Simple)
It is so hot that we cannot go out without an umbrella. (Complex)

Seeing the snake, the boy shouted.
When the boy saw the snake, he shouted.

We didn’t go out because of the rain.
We didn’t go out because it rained.

By expanding a word or phrase into an adjective clause

A simple sentence may be converted into a complex sentence by expanding a word or phrase into an adjective clause.

Prudent boys work hard. (Simple)
Boys who are prudent work hard. (Complex)