Gen Alexandra said:
Agentive particles also mark the subjects of the verbs of uncontrollable perceptions ཤེས་, མཐོང་, ཐུག་, གོ་, ཧ་གོ་, མཇལ་, and knowledge in all tenses.
Example: ངས་ཧ་བགོས་སོང། ངས་ཁོང་ཐུག་གི་འདུག ངས་ཙ་ལག་གོས
I could not find it in The Heart of Tibetan Language Vol. I, so I was confused about that use.
However Dimitri’s logical reasoning convinced me:
When you use non-voluntary verbs of perception (I see something, I hear something, etc.), they are usually involuntary and you always have the subject and the object. At the object is NOT marked by a la-particle. When you say “I look at something” (using voluntary verb ta) the object is marked with a la-particle, so there is no source of confusion. But with those verbs confusion is always possible. Thus, you always have to mark the subject with the agentive particle.
I would also hypothesize that not only involuntary verbs of perception, but all transitive verbs (but only where the object is not marked by a la-particle) would also require the agentive particle for the subject - to avoid confusion. For example: I eat something. Or: I hit something (or somebody). Or: I send something or buy something. But it’s just my hypothesis. It would be nice to hear what gen-la thinks.
Gen Alexandra la agreed and also expanded with Classical rules:
All agents of transitive verbs need to be marked with an agentive particle in Classical Tibetan, including “transitive” verbs whose object takes a ལ་དོན་ (now do you see why I avoid using the categories “transitive / intransitive”?). Sometimes the agentive particle also marks an object:
བུམ་པ་ཆུ་ཀྱིས་གང་། The vase (subject) is full of water (object).