Just a suggestion.. This Verbal Noun is WAY too complicated for one set of lessons. There should be at least three sets - one introducing the format of all the participles, just I am coming, they are going, he is collecting, etc. So we learn the verbal noun forms. Then one to introduce its use as "to come", to go, to collect, etc. with nouns as objects. Finally one introducing the forms with pronouns as the objects. Having them all lumped together in the same lessons is very confusing.
I read all the grammatical notes caregully ( but only twice which is not enough) but I cannot find anywhere when you use "ag" before the verb/ gerund and when you need only "a" without a pronoun fir example in the sentence "an bhfuil tú chun carr a oscailt di". Can someone explain? Thank you
The Tips & Notes are not a core part of Duolingo - most Duolingo users don't even know that they exist. They are simply an add on that allows some learners to shortcut the learning process that Duolingo is designed to use, staged repetition with increasing exposure to new material.
So the Tips & Notes are not meant to be comprehensive - you're supposed to be able to figure out the difference between ceapaire a ithe - "to eat a sandwich" and ag ithe ceapaire - "eating a sandwich" on your own.
If you need more detail, you need to turn to a resource that is designed to teach the grammar by describing it, rather than using examples to demonstrate it, such as Gramadach na Gaeilge.