Auxiliary missing

Hi
It seems to me that auxiliaries seem to be left out at times in Swedish and in a sentence they might just say kunnat instead of har/hade kunnat. At first I thought I was mishearing things but i have now seen quite a few examples of missing auxiliaries. I have spoken to a Swedish colleague and he had never noticed it and could not explain it. I have looked in a number of Swedish language-learning books and also found nothing. Is it the perfect participle that is being used, or is it something like it but different…? Has anyone else noticed this…? What verbs does this happen with? What are the rules? How is it formed? This is not, as far as I can tell, because the auxiliary was already used earlier in the sentence either, as in English where it is not repeated if it has been used once before.

I’ve noticed this, too! Here’s a link: Swedish: omitting the auxiliary verb in dependent clauses | WordReference Forums

I guess it has something to do with dependent clauses. I’ve noticed this in spoken Swedish, though in informal written correspondence I imagine it would also be permitted.