Combination of two or more simple sentences into a single complex sentence
Make one of the simple sentences the principal clause and convert the others into subordinate noun, adjective or adverb clauses.
By using a noun clause
A good education is essential for success in life. I believe this.
I believe that a good education is essential for success in life.
Can we depend on our guide? We don’t know.
We don’t know whether we can depend on our guide.
Note the word order in clauses introduced by whether and if: the subject comes before the verb.
I don’t know if I am correct. (NOT I don’t know if am I correct.)
You are bent on mischief. It is known to everyone.
That you are bent on mischief is known to everyone. It is known to everyone that you are bent on mischief.
Virtue is its reward. This is certain.
That virtue is its reward is certain.
There should be no spelling mistake in your writing. Always remember this.
Always remember that there should be no spelling mistake in your writing.
He said something. That is of no significance to me.
What he said is of no significance to me.