classes

If the three items below are classes, how do you know which ones to use and how many of them are there? Can you use all of them with one command?

using System;
using System.Collections.Generic;
using System.Text;

yes, we can have them one by one

visual studio will give you an error and hopefully a popup showing what is needed.

these just come with experience, the more you work in the software the more you will simply know which using statements are needed for which jobs.

not point in memorising them all if you only program within one certain field.