With the third conditional, Elena, we are often talking about something that might have happened, but didn't. As your example illustrates, when we are discussing such past situations we normally ...

As a result, using ‘have’ in place of had in that particular conditional clause would be totally wrong just as the choice of the tense in the main cause it is introducing should be ...

Meaning and use We use the zero conditional to talk about things which always happen if a particular action takes place. These can be scientific facts, or things we believe are always true.