What is the difference between them? Are these sentences grammatically correct?
- What will happen in the world?
- What will happen to the world?
Feel free to correct any of my sentences, because I want to learn a lot from my mistakes.
They are both grammatically correct.
If, instead of "the world", we asked about "the house":
The question with the word in would be asking about what events would take place within the confines of the house.
The question with to would be about the fate of the entire house.
But in this question, "the world" is very vague. If it refers to all of humankind and their environment, both questions mean about the same thing.
Yes, there is a difference between them.
What will happen in the world?
This is much more generic and does not mean that what will happen to the world as a whole, but a specific place/part of the world.
For example, something could happen in the USA or the UK or Asia.
What will happen to the world?
This is much more focused on what will happen to the world as a whole as apart to specific places in the world.
For example, a World War.