Third Conditionals are sentences with two clauses – an ‘if’ clause and a main clause – that describe the past. They are used to describe ‘something that didn’t happen’.
We would have arrived on time if we had left earlier.
We left late so we arrived late.
If I had studied harder at school I would have gone to university.
I didn’t study very hard so I didn’t go to university.
Caroline would have come to the party if you had told her.
You didn’t tell Caroline so she didn’t come but if you told her she would have come.
The third conditional is usually formed with if + past perfect and would + perfect infinitive – have arrived.
Other modal verbs can also be used instead of would – (might, may – could)
In mixed conditionals, the time between the first and second clause is not the same.
We would be there now if we had left earlier.
We are not there now because we left late.
If I had studied harder I could have a better salary.
He didn’t study hard (in the past) and now he has a low salary.
Caroline would be coming tomorrow if you had told her about the party.
Caroline will not be there tomorrow because you didn’t invite her. (in the past).