"You have six grandfathers."
Translation:Tá seisear seanaithreacha agaibh.
Singular nominative fir most things, plural genitive for people from 2 to 10 and 12
You do not need to! A lot of people just ignore most of these rules, and eventually some of them are doomed to disappear in future official standards! No one will misunderstand you if you count people like thinks... Eventually you will get it, but it is not a priority!
Singular nominative after Cardinal numbers (and cardinal numbers are adjectives in a phrase like "six grandfathers").
"Seisear" is a noun, not an adjective, and is not a number, per se - it's a noun, and is closer in function to "sextet" than the number six, (though "seisreád" is the Irish for "sextet"), and takes the genitive because it's essentially "a seisear of grandfathers".
I've found this — "sextet of" or "a trio of," etc. — a very helpful way of thinking about the numbers for counting people. Thanks, SatharnPHL!
Please remind me: do we use the singular for things, but the plural for people? (it's probably in the tips and notes, right?)
Singular nominative for things. Genitive plural for people (a dozen "of" men).
I wrote 'tá seisear seanaithreacha agat' and got it correct. I then came into the comments and it says 'agaibh'. Should I learn it as agaibh so as to not sound like an idiot, or are they equally correct for this type of sentence?
If you're talking to more than one person, you say Tá X agaibh, if you're only talking to one person, you say Tá X agat.
Normally, a single person only has 2 grandfathers, but throw a couple of divorces into the mix, and someone could have 6 grandfathers, so both tá seisear seanaithreacha agat and tá seisear seanaithreacha agaibh are both valid, though you are more likely to be talking to 3 people when you are talking about 6 grandfathers, so tá seisear seanaithreacha agaibh is more likely.