"I am seeing a big grandfather."
Translation:Tha mi a' faicinn seanair mòr.
You use "seanair" when talking about the grandfather. When you are addressing him directly it has to turn into the vocative case which is "a sheanair".
There is a basic explanation of this in the Tips and Notes for the "Phrases" skill (the second one on the tree), and then a much more detailed explanation further down the tree with the "Names" skill (the 19th skill).
I don't think you can really understand this course if you don't use the notes. I started a Gaelic class last week and a bunch of people were telling the teacher how you just had to guess until you hit the right answer and I was thinking that's not right I've done the whole tree and I wasn't guessing, but they didn't even seem to know about the notes.
If you're having trouble accessing them from the individual skills, the whole lot are here on one page for easy reading. https://duome.eu/tips/en/gd
In additon to what Morag Kerr said, you also use "sheanair" after the possessive pronouns "mo / do / a" (my / your (singular) / his), although not after the others ("a / ar / ur / an" - her /our/ your (plural) / their), and after the few adjectives which precede, rather than follow, a noun.