If I wanted to say 'he is an author' rather than 'he is a writer,' would the 't-' still stand before 'údar'? So, would I write (for example 'Údar is ea í' or T-údar is ea í'? And if the latter, what is the 't-' for?
And if I was to say 'I am' (etc) would the pronoun be 'mé'? I think it would be, but given the other pronouns (sí, sé, síad) lost their first letter, becoming 'í,' 'é' and íad' I wanted to make sure.
The t- is used before singular masculine nouns that begin with a vowel following an not governed by a preposition, so “He is an author” would be Is údar é, “She is an author” would be Is údar í, and “I am an author” would be Is údar mé.
The historical reason for the t- can be found here.
That's rather circular logic. Obviously, if you drop the o from scríobh, you have to change the ending from óir to eoir. If the o is "only there to show that the "bh" is broad", why isn't it kept broad by using a óir ending?
In short, how come the verb scríobh has a broad bh, but the noun scríbhneoir has a slender bh?
No doubt the n plays a role, somehow.
I can't reply directly to your comment - Duolingo restricts nested replies to 5 or 6 levels.
There is a rule when spelling words in Irish, *leathan le leathan agus caol le caol". Broad with broad, and slender with slender. In it's simplest form, the vowel following a consonant must be the same "width" as the vowel immediately preceding it.
If the vowel before a consonant is e or i, then the vowel following it must be e or i. If the vowel before the consonant is a, o or u, the first vowel after it must also be from a, o or u.
So, for example, you have múinteoir for teacher, but léachtóir for lecturer - the i before the nt consonants must be followed by an i or an e, and the a before the cht cluster must be followed by an a, o or u.
The "width" of the consonants can change the pronunciation too, in some cases.
In a normal sentence, the verb comes first, and an before a verb is the interrogative particle that turns a statement into a question, so when a sentence starts with an it's usually a question. But scríbhneoir is a noun, not a verb, and an before a noun is usually the singular definite article "the".
The obvious exception to this is a copular classification question - An scríbhneoir thú? - "Are you a writer?", because the interrogative form of the copula doesn't use an interrogative particle, it's just an. But this sentence isn't a copula - in fact, you could argue that it isn't really a sentence, because it doesn't have a verb, a subject or an object!
So in this case, the an comes before a noun, and it's just the singular definite article.