A reader sent me something from a newspaper and said that it sounded incorrect but that she couldn’t quite explain why. The sentence in the newspaper was If the house would have been newer, it would have been demolished. I tip my hat to the reader for immediately recognizing a grammar error when she saw one.
This is a case of an incorrectly constructed conditional sentence. In these types of hypothetical sentences, we need the conditional perfect in one part of the sentence and the past perfect in the other. The conditional perfect is used to indicate that something would have happened if something else had been true.
In If the house would have been newer, it would have been demolished, the speaker is using the conditional perfect (would have been and would have been) in both parts of the sentence, but, as I said, it should only be in one part of the sentence if you want to be grammatically correct.
When constructing a conditional perfect sentence, use the past perfect for the if clause and the conditional perfect for the main clause. In our example sentence, then, we would have to change it to If the house had been (past perfect) newer, it would have been (conditional perfect) demolished.
For those who want more information, here it is—–but don’t say that I didn’t warn you.
To form the past perfect in the if clause, you need had and the past participle of the main verb. For example, If he had known… Here, you have had and the past participle of the verb to know, which is known.
To form the conditional perfect in the main clause, you need would and the perfect infinitive of the main verb. (The perfect infinitive is just have and the past participle of the verb.) For example, He would have stayed. Here, you have would and the perfect infinitive, which is have stayed.
If we put them together, we get He would have stayed if he had known or If he had known, he would have stayed.
It’s the same for our newspaper example: If the house had been (past participle in the if clause) newer, it would have been demolished (conditional perfect, which is composed of would and the perfect infinitive have been).