"Is Fergus there?"
Translation:A bheil Fearghas an sin?
Because in last question (whichever question that was for you) apparently it was in a vocative (the question directly addressed Fergus, speaking to him) while in this one Fergus is the subject, in nominative (the question is about Fergus, not addressing him directly).
You can read about using vocative in Gaelic in the lessons tips and notes – they are available in the web version of Duolingo but not in the Duolingo mobile app (available when you access Duolingo using a web browser – mobile also work, but not in the app itself). You can also access the notes at https://duome.eu/tips/en/gd
There's a couple of different things going on. "Seo" and "sin" by themselves mean "this" and "that", whereas "an seo" and "an sin" are "here" and "there". That just applies when "an" is directly next to "seo" or "sin" though - if you see them separated it will be the this/that meaning. e.g. "an (noun) seo" or "an (noun) sin," which is how you'd say "this specific thing" or "that thing."
The basic form (the nominative case) of the name is Fearghas – you use it when speaking about Fergus – what he does, what happens to him, etc.
Fhearghais is the vocative case – used when you address Fergus directly (eg. ciamar a tha thu, Fhearghais? ‘how are you, (o) Fergus?’), and genitive case, English Fergus’s, of Fergus – used when speaking about things belonging to Fergus (eg. taigh Fhearghais ‘Fergus’s house’).
You can access the tips and notes to the lessons on the web browser version of Duolingo at https://duolingo.com and also on the https://duome.eu/tips/en/gd website. They might not be available in the Duolingo mobile app – so if you are on a mobile device, you might want to open your web browser for the reading.