"Ann an oilthigh."
Translation:In a university.
There are two things that catch people here.
Sometimes an means 'the' and if it did here, you might wish to insert a t-. But here, ann an means 'in' (without a 'the' - 'in the' is anns an, a confusing but important difference).
Even if id did say anns an (as I misread it at first) then there is still an issue. They are trying not to introduce too many rules all at once. They prefer you to get a feel for what is right, so you can get used to it, just as children learn without any rules.
You will later discover that most prepositions are followed by the dative case. To oversimplify, this means that masculine singular nouns behave as if they are feminine. And there wouldn't be a t- if oilthigh were feminine, would there?
Learning by example is great, but spotting when things are different is an important part of that. So well done for spotting the lack of t-, but you also need to spot the lack of s! D
I asked about this on another thread: https://forum.duolingo.com/comment/35769178?from_email=comment&comment_id=41118869 at would generally be translated by "aig"