Why won't my program work?

Here is a checklist of things to look for if something is wrong…

Remember that the computer is really, really, really, really stupid. It will recognise only words it has been taught; it is not clever enough to handle spelling mistakes - these will confuse it. It is also not clever enough to know that most of the time we use A and a interchangeably, so you have to make sure that if the computer is expecting a capital letter, you give it one, and vice versa.