A conditional sentence is a complex sentence with a subordinate clause of condition that usually begins with the conjunction IF. The clause of condition (the if-clause) indicates the conditions under which the action in the main clause may be realized.
Third conditional (Such sentences describe imaggined past situations and the consequence of these situations.These consequences and situations are the opposites of what happened.
If+Past Perfect in suboding clouse , would + have+ past participle in principal clouse
If I had gone to bed late, I would have been tired.
Mixed type of conditional sentences
In such sentences we speak about past and present
It can be a past tense in the if-clause and would have+past participle in the main clause or past perfectin the if-clause and would +infinitive in the main clause
If Bob weren't so lay, he would have passed the exam easily.
If I had got my visa last week,I should be flying over the Atlantic now.