8.2.4 Formation of the plus-que-parfait du subjonctif
Also the form of plus-que-parfait du subjonctif is only used in written language. Might be the reason that we need the imparfait de
subjonctif of the verb avoir (for transitive verbs) and imparfait du
subjonctif of the verb être (for intransitive verbs), and the imparfait de subjonctif is only used in written language. However, if you know them, the tense is easy to be formed. In addition to the e imparfait de subjonctif of the auxiliary the well know perfect participle is added (with the respective suffixes for intransitive verbs as we already know from the other composed tenses).
Transitive
Verben
introductory
verb
conjunction
personal
pronoun
auxiliary
verb
perfect
participle
completion
Je
craignais
Tu craignais
Il / Elle craignait
.....
Je voulais
Tu voulais
Elle voulait
Nous voulions
You probably have noticed that the introductory verb is in a past tense (imparfait, passé simple, plus-que-parfait*) before the plus-que-parfait du subjonctif is used. This lies in the nature of the past perfect, that it is an action that happens before the time of narration.
* Actually here we should have also mentioned the passé composé
as a past tense. Officially it does not count to be one, practically it is used as one. Here we can see the difference between theory and practical experience.