"They are thirsty."
Translation:Tha am pathadh orra.
It's a sound we don't have in English but it may sound like g because that is the closest sound we do have. If you have a knowledge of phonetics it is easy to describe. It is /ɣ/. It is described in https://en.wikipedia.org/wiki/Voiced_velar_fricative. If you happen to know any of the languages listed there then that will help.
But if you don't know phonetics then it is hard to describe. It is the voiced equivalent of broad Gaelic or Scots ch. That means that your whole mouth does exactly the same as for broad ch. The difference is that your vocal chords vibrate. That makes it more g-like than c-like. I hope that helps as it is the best I can do without going into the phonetics.
There are two possible questions here:
When do we use 'the' in Gaelic? It is not possible to list all the situations that differ from English here, and the best thing is to get used to the situations rather than trying to learn strict rules, but general concepts such as 'thirst' do tend to use 'the'.
When is 'the' am in Gaelic? In the very simple sentences encountered so far, you use am for all masculine singular nouns that begin with a sound made with the lips (because then it is easier to say am than an). These are b,m,f,p.
Well they have obviously told Duolingo to accept both. It depends what you mean by 'correct'. Am is traditional and it is what all grammar books will tell you - thus it is what learners do. I think it is younger native speakers who most commonly use the English-influenced form without the am. There are at least two places in this course where they are actually teaching the English form to the bafflement and annoyance of lots of people.