So the answer to your questions:

1. Is answered in the parent comment

2. It's not really about solving problems directly. Its more about a common way to hold meta information about your data. But of course that might help you with approaching similar problems.

3. Thats hard. You dont really think about it that way. Again its not really the type of problems directly. Its more about similar approaches to solving different kinds of problems. So you can use monads for solving basically any problem. But you can also choose to not use monads to solve the problem. This is something you'll need to get a feel for. And it shouldn't be for first line of thought. When solving a problem you might recognize that a certain monad lends itself to solving the problem. But it's not really something you actively go looking for.