A truly nightmarish rule. After the preposition i, you eclipse. I dteach. After the article plus i, you lenite. Sa chathair. But the letters d, t and s are exceptions to this rule. Sa teach, sa dorchadas, sa spéir. Sin é mar is dóigh liom, in ainneoin nach bhfuilim féin as an ghaineamh súraic, go dtí seo..
This one is a dirty trick. The Irish phrase doesn't include the definite article, so you figure you don't need it, then you get nailed for omitting it. In the question where you're supposed to translate English>Irish you have "the tea" so you figure you need the definite article - and get dinged for including it....