Relative clause

A "relative clause" is a string of words that contains a noun and a verb. Relative clauses can not stand on their own as a sentence and must begin with a "relative pronoun".
This is one of the most difficult aspects of English. Consider:
Consider these two:
On the first, he will make a large profit, and it gives the reason.
In the second, the profit only comes if he builds well. The profit is "conditional" upon his performance.