4. Defining your own behaviour of existing operators on your own types. (For example, you could define a Car class and then define what it means to add two car instances).
5. Yes. (In C# you can, in C you can't, so it actually depends on the language).
6. An array is a sequence of elements of the same type in memory that you can reference using an array index.
7. Depending on the language, the string type can have a Lenth() property (c#). Other languages can have library functions such as strlen().
8. If your data represents something multidimensional, like when you want to model chess pieces on a chess board.
9. In some languages you will corrupt some memory, and your program behaviour will become unpredictable. In other languages you will get a run-time exception because the memory is protected.
10. Those variables are typically dummies, meaning, they don't have a strong meaning. i is short for "index", and j and k are simply the next in the alphabet.
