We have a few lines that imply this in the tips and notes for the Eclipsis skill - but we probably don't make it explicit enough.
If the word begins with s, it is feminine, the next letter in the word is a vowel, l, n or r, and you are using the word after the singular definite article an, then a t is placed in front of it.
- an tseanbhean with the old woman
If the word begins with s and is masculine, no change occurs.
- an salann with the salt
The t- with a hyphen is used after the word an:
- for all singular masculine nouns that start with a vowel (except for euro, oiread and iomad)
- for the words aon, aonú, ochtó, ochtú and ochtódú.
We must find an appropriate place to put these notes in the course!
Edit: apparently we already slotted this into the Dates and Time skill! (the first place you encounter the t-prosthesis)
Alright. You give an initial t, no dash, to feminine nouns starting with s, followed by a vowel or l, n, or r, if the word is preceded by 'an'.
You put a t- in front of a masculine word if it starts with a vowel (with some exceptions) and is preceded by 'an'. (But I recall that some prepositional phrases cancel this, as is 'ag an aerfort'.)
Thanks, I think I have it.