I had this question the last time I did PPs and still do.
The AuxVerbs, such as Ser and Ficar, are in the Past Perfect tense.
Ter is in the Past Imperfect.
Why?
Can you give more details? Maybe a link or example. I think it depends on the context.
Learning Notes from Past Participles (B.1)
Thank you
I see what you are saying now. I am sure the native speakers will be able to comment. I never noticed that detail!
I just figured it was just supposed to be that way. i will await a real answer.
Thanks. That’s my best guess too: It’s always been done this way.
@sbaratto, this has to do with the roles these auxiliary verbs play. When paired with past participles, the verb ter (and also haver, although mostly out of use) is always used to form compound tenses. Among the existing compound tenses, you’ll find that the only options for ter are the simple present, imperfect, future, conditional or infinitive forms, as shown in the following Learning Notes:
- Compound Tenses in the Indicative and Conditional Moods – Practice Portuguese
- Compound Tenses in the Subjunctive Mood | Practice Portuguese
- Compound Tenses with Nominal Forms – Practice Portuguese
With auxiliary verbs such as ser and ficar, the past participles are used more like adjectives. You’re not forming compound tenses with them, so they don’t have the same tense/mood restrictions and can adopt different forms 
Thank you, Joseph.