A typical type 3 conditional sentence usually comes in the form of “If + past perfect, would/wouldn't have + past participle”.
The past perfect tense is used because we are mostly talking about more than one thing.
Example: “The karate class was fun, if Mike had been there, I would have shown him a few tricks.”
In this example, for the speaker, “if Mike were there...” was referring to the karate class (the “there”) that has already ended in the past, thus, in order to clear up which event happened first in the past, past perfect tense is adopted.
Simply writing a conditional sentence with past perfect tense does not make a type 3 sentence since perfect tenses are used to talk about activities that are within the realis aspect.
So, there needs to be one more factor to turn the realis aspect into an irrealis aspect, which is including the adverb “would” + “have”.
“Would” is a modal auxiliary verb, the word itself is entirely in the irrealis aspect, according to Givon, the presence of such an irrealis operator within a clause overrides the otherwise realis value of past, present or perfect. Which explains the reason that despite using past perfect tense in type 3 conditional sentences, the whole event remains in an irrealis aspect.
