"This dish is lovely."
Translation:Tha an soitheach seo àlainn.
Seo [x] - This is (a/an) [x]
Seo an/am/an t-/a' [x] - This is the [x]
Tha seo [x] - This is [x]
Tha an/am/an t-/a' [x] seo ... - This [x] is ...
Remember, Gaelic distinguishes between inherent state, and temporary state; "this is cold" = 'tha seo fuar' but "this is a fish" = '(Is) seo iasg'.
To slightly further the above. The an in here kind of isn't meaning "the". Although of course, that is what an usually means.
An is a definite article. It is the dish as opposed to a dish. It's usage here is to make certain that the dish in question is the dish which is lovely, as opposed to some other hypothetical dish.
And by the way soitheach means dish in the same sense it does in English. It's both used as a bowl and as a word for a meal.