"James has a t-shirt."
Translation:Tha lèine-t aig Seumas.
There is an error with the sound here. Sometimes in Gaelic, t is just a letter added to the beginning of a word and it is pronounced as a letter. The speakers attempt this, although it is difficult when the letter is on its own.
But this is different. This a t-shirt. t is pronounce tì.
In your Frog example I understand it as ... I have a frog = Tha losgann agam. You have a frog = Tha losgann agad. He has a frog = Tha losgann aige. James has a frog = Tha losgann aig Seumus.
This is slightly different obviously because it's a t-shirt not a frog - but the rules still apply!
I'm still a beginner so please someone correct me if I'm wrong.
That is exactly right. For completeness, here are all the forms of this prepositional pronoun that are used to say 'I have', etc.
|Prepositional Pronoun||English Translation|
|agad||at you (singular)|
|agaibh||at you (plural/formal singular)|
(copied from https://duome.eu/tips/en/gd#Phrases-3)
You will have to learn this set for each of the simple prepositions, so congratulations on your first one.
And you are quite right that it works the same way for frogs and T-shirts and everything else. You never know what is suddenly going to change that you are not expecting, when you learn a new language, but there is no problem here.
You are, of course, correct - aig is a preposition. The confusion may arise from the translation of aig at https://duolingo.fandom.com/wiki/Scottish_Gaelic_Skill:Colours which says it means "has".
That is a good point. That is not the only website that thinks that translation is helpful, and you can see why someone might think it is helpful, but the evidence is in these pages. So many people here are totally confused by it and misunderstand what all the other words in the sentence are doing.