"He is working in an office."
Translation:Tha e ag obair ann an oifis.
ann an / ann am means "in" rather than "in the" - it's a good idea to try to think of ann an/am as one word, to avoid confusion with an/am as "the": ann an in oifis an office.
"In the office" would be anns an oifis. Duolingo doesn't introduce this form at this stage, as there is a complication (the noun has to be in the dative case, which is not an issue for a feminine noun like an oifis, but is an issue for masculine nouns).
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There are three reasons for not using t-oifis, some of which will only become clear later in the course.
an t- is only used as a definite article before masculine nouns beginning with a vowel, and only when they are used in the nominative case - oifis is a feminine noun, and means "an office" rather than "the office", which would be an oifis. So t- only occurs as part of a masculine definite article, which leads to 2.
The an/am in ann an/ ann am is not the definite article - it's best to think of ann an/ ann am as a single unit meaning in, i.e. as [ann an][oifis] rather than [ann][an oifis] - that should help make clearer that it means "in an office" rather than "in the office". ann an/ ann am doesn't vary according to number or gender, and only varies from ann an to ann am before words beginning with b, f, m or p - i.e. it doesn't vary from ann an before words beginning with a vowel.
even if we were talking about a masculine noun beginning with a vowel, e.g. ospadal, "a hospital", an t-ospadal, "the hospital", the word would never appear with the t- in phrases meaning "in a/the hospital". In the case of "in a hospital", there is of course no article as Gaelic doesn't have an indefinite article - [ann an] [ospadal]. In the case of "in the hospital", the noun and article have to go into the dative case (sometimes called prepositional case) - this only comes later in the course. Masculine nouns beginning with a vowel with the definite article change from e.g. an t-ospadal in the nominative case (i.e. where they are the subject of a sentence) to an ospadal in the dative case - "in the hospital" would be anns an ospadal (anns an/a'/na for "in the" is also introduced later in the course - here the an/a'/na does represent the definite article).
The short answer is that t- will never appear before words beginning with a vowel in phrases involving "in", whether with ann an/ ann am or anns an/a'/na.