Every time I see "an t-ubhal" I think of what is probably my favourite Runrig song, "An t-Ubhal as Airde" (the highest apple) - https://www.youtube.com/watch?v=K62sxZw-Qes
Mi cuideachd. Tha òran cho math!
Why is t here?
Definite masculine noun in nominative (standard) case that begins with a vowel = "an t-…" as the definite article.