The names of many countries need a definite article in Gaelic where it's not needed in English. In English we use "the" for a very few countries (e.g. the Netherlands, the Gambia, the Philippines, the Bahamas) - in Gaelic we use the definite article for far more countries. This is explained in the tips for the unit. All the tips can also be found here: https://duome.eu/tips/en/gd
The difference between a', an and an t- for feminine nouns is also explained in the tips for Animals or in the Duome link - the form of the definite article depends on the first letter of the noun that the article is used with. A bit like how a and an mean the same thing in English, and are used depending on the first letter of the following word. It's important to learn these rules now, as they will be used more and more as the course goes on. They should eventually start to feel like second nature, even though it all feels complicated at first.