So I can see how once we know our declensions we are able to say that "scoil" is 2nd declension, and we can follow the rules to create a genitive singular.
If we knew by heart the genitive plural was "scoileanna" we could deduce that this phrase could only be in the genitive singular.
However if we didn't know the plural forms by heart, how can we be certain that "scoile" is genitive singular? There doesn't seem to be any rules for forming genitive plural in the 2nd declension.
I've read up on weak and strong plurals here https://en.wikipedia.org/wiki/Irish_declension#Nouns but that doesn't seem to help as its only a comparison of the nominative and genitive plurals after they've been formed and doesn't give the clue as to how to create them.
Even if we could tell if a noun was a weak plural or a strong plural from looking at the common/nominative form, that could help in some of the cases (for strong plurals, as long as we knew the nominative plural).
something i'm missing here? or just a case of learning them off
I know there are some words in 2nd declension that have standard endings, and there is a table for these, but what of the ones that don't...?
If you know that scoile is of the 2nd declension, then you can be certain that scoile is not nominative singular, because all 2nd declension nominative singular forms end with a consonant. Because 2nd declension nouns take their genitive singular forms by either (A) appending -e to a final slender consonant, (B) slenderizing a final broad consonant and then appending -e, or (C) changing final -each to -í and final -ach to -aí, one can conclude that scoile is a genitive singular form. However, one couldn’t be certain that it must be the genitive singular, since some 2nd declension nouns have identical genitive singular and nominative plural forms, e.g. súile for súil. One would need to know that scoil is both 2nd declension and that it has a strong plural to be certain that scoile is only its genitive singular form, since if it has a strong plural, its genitive singular form would not be identical to its plural form.
How to identify 2nd declension: 1). mostly f and 2). ends with a consonant (broad when in Connacht)
(Hence it's helpful to learn each noun together with its gender. (Btw. I really wish DL would show the gender when you hover over a noun).)
Distinction from 3rd declension (since that may also meet two criteria op. cit.):
gs. of 2nd decl. ends -e, whereas of 3rd declension ends -a. npl. of 2nd decl. often ends -a, whereas of 3rd decl. ends -í.
So remembering gs. / npl. can help determine declension.
I don't really see this difference. If you're talking about the school cat, in idiomatic English, you are talking about a particular school, otherwise it would just be a school cat, i.e. the cat of a (some, undefined) school. Just as if I send you to the school principal, I'm not talking about the principal of any old school, but the one in which the statement is made.
I would agree with you. I had thought of putting "the school cat" but thought DL would reject it.
You could think of various other similar examples where, when talking about a specific school etc we wouldn't, necessarily, use a genitive construction: "I am in the school choir/band/team", "She works in the school/school's library" etc.
Or this example from a book (Dead Set by Jennie Melville): 'It was only the cat, the school cat.' The school cat was a strongly marked tabby. Here, I would say, the reference is to a definite cat and a definite school and, I think, "school cat" corresponds to "cat na scoile".