That's not entirely true. I can say, "Penso che io l'abbia lasciata qui," and that's a perfectly good use of the subjunctive. The subjunctive generally requires that you re-specify the subject when there's no context, though. So with this sentence, it could be "he/she/I/you/it".
I can say, "Penso che io l’abbia lasciata qui"…
That is a very unlikely sentence.
The subjunctive generally requires that you re-specify the subject when there’s no context…
Only in the second person singular.
So with this sentence, it could be "he/she/I/you/it".
No, it could only be he, she, or it.
As Viaggiatore showed, when the subject of both clauses is the same, you would more frequently use "di + infinitive" instead of "che + subjective". It's not something Duo actually teaches (as of this writing in Oct 2015) but you will find many people discussing it in other threads.
Yes, you're correct about that. But we'd need a context or some modifiers.
The trouble is that English verb conjugations are very vague compared to Italian. So just as a stand-alone sentence, we'd probably have to use the ongoing "was using" form of the verb to be clear on the tense.
Down with your cynicism! The time may come to you, when after a ( quire probably bizarre) event in Italy, you will be questioned and you will be ready, to stand up say in a strong clear voice (with correct pronunciation/accent) 'Credo che abbia usato un cuscino!' and you will be thankful then to DL then!.
Because (unless I've missed something), I have no idea who you are and why you consider yourself to be an authoritative source on the Italian language.
Who says that I consider myself an authoritative source on the Italian language?
But please feel free to link to an explanation of your position on this subject.
OK, what do you want to have explained?