In Orberg LLPSI he has: 'Marcus magistrum scribere animadvertit eumque interrogat: “Quid tu scribis, magister?”
I would have written this ‘Marcus magistrum scribentem animadvertit…’ Why is ‘scribere’ in the present infinitive? Surely it’s a participle and should be written ‘scribentem’?? Or is this some version of infinitive + accusative…but it doesn’t make sense to me because that’s only used when the action is being reported or commented on by a third person: Marcus ‘magistrum scribere’ dicit.
Mārcus magistrum scrībere animadvertit: “Marcus notices that the teacher is writing”
Mārcus magistrum scrībentem animadvertit: “Marcus notices the writing teacher”, or maybe “Marcus notices the teacher as he writes”
Does this help? Compare e.g. “Videō tē scrībere”, “I see that you are writing”.
The acc. + inf. construction isn’t just for speech, but also for various types of perception and mental action. It’s found pretty much anywhere where in English you say “he [verb] that”, e.g., he saw that…, he heard that…, he imagined that…, he thought that…, he knew that…, he hoped that…, he discovered that…, he threatened that…, he promised that…, etc. It’s very common.