Conditional clauses. As you have seen, there are five common types of conditional sentence. Conditional sentences are made up of two clauses; the ‘if’ clause and the main clause. When we start the sentence using ‘if’, then the two clauses are split by a comma. We can also start the sentence using the main clause. If we do this, we do not include a comma. In 1st, 2nd and 3rd conditional sentences, the main clause contains will, would, would have, or another modal verb, as we will see later. It is very important to note that ‘If’ and the modal verb never appear in the same clause. This is a very common error. Here are some examples.