Uk political system
The United kingdom and Northern Ireland

В зависимости от контекста. Время не влияет на артикль.
Если ты говоришь о кенгуру в обобщающем значении, то неопределённый артикль a, а если в конкретизирующем значении, то определённый артикль the.

A kangaroo is one of the most unique animals in the world. (Кенгуру является одним из уникальнейших животных в мире)

The kangaroo seems to be hungry. (Кажется, этот кенгуру голоден)

В выражении "What a nice kangaroo" всегда неопределенный артикль, в выражении "How nice the kangaroo is" всегда определенный артикль.

I used to wear glasses when I was at school, and so I found sport very difficult. I wanted to be involved in school sports competitions, and I loved the idea of teams competing with each other, but being on the rugby team, for example, meant having to take off my glasses, and that meant that I couldn’t see! And it’s difficult to catch a ball when you can’t see it! So, I wasn’t very good and the captains always made sure that I wasn’t on their team. This meant I was free to do other activities, like being on the debating team, and actually I preferred to do things I was good at doing rather than have everyone laugh at me.