What's the main idea of "The Third and Final Continent"?
In general, if you need to express the main idea of a story, you can do it in this template: "Somebody wants _____, but _____, and the end result is _____."
Put another way, you could explain the main idea of the story by telling who the main character is, what that person is trying to do, and how it all ends up.
This method works because, at its core, every story is about a struggle or a conflict.
So for "The Third and Final Continent," let's say that the main idea is this: The narrator wants to have a happy marriage with his new wife, Mala, and make a smooth transition from life in India to life in the US. But he worries that his arranged marriage will make things awkward and difficult, plus he feels isolated in his new social environment. The end result is that his landlady provides him with the friendship he needs and the inspiration to see Mala as someone worth falling in love with.
I should also mention that when some instructors say "main idea of the story," they don't mean "what happens." Instead, they mean "what we learn from the story" or "what the story illustrates about life."
If that's the case, then we can find that main bigger idea, or theme, by considering what the author was trying to express by writing the story. What does it reveal that's true about life, or love, or society?
For "The Third and Final Continent," let's say that the main bigger idea is that friendship and understanding can ease a difficult transition. More discussion of the themes of the story can be found here.