"I like this supermarket."
Translation:Is maith liom an t-ollmhargadh seo.
The Article (an) prefixes "t" in the common form to masculine singular nouns beginning with a vowel except when governed by den, don, san, ag, ar, as, chuig, faoi, le, ó, roimh, thar, trí, um. For example "Cad é an t-am é" (what time is it). Tá an t-ádh leat (you are lucky) : an t-arán (the bread), an t-ainm (the name), an t-éan (the bird), an tEarrach (the Spring). But note :- ag an am, leis an ádh, san arán, ón ainm, ag an éan, etc. There are other rules to prefix " t" but the above is the answer in this particular case.
Technically speaking, the nouns governed by those prepositions are in the dative case (an tuiseal tabharthach), and (with very few exceptions), the form of the dative and the nominative are the same, but there are different rules for applying mutation in the dative. For example, the gender of a noun is a significant factor in the mutation of nouns in the nominative, but gender doesn't effect mutations in the dative.