Answer to question 1:
Humans understand human languages such as English, Spanish, and so on and so forth. Computers do not.
Computers can understand low level languages, often called "machine code", which are basicaly a very ackward set of numeric values that represent instructions to the computer. Humans can not.
The solution: a compromise. Programs are written on high level programming languages like C# or Visual Basic which resemble the languages that humans can uderstand (Most are based on the english language), and afterwards, they are "compiled" (translated) into machine code so that it can then be executable on a computer. Once compiled (translated into machine code) a program is no longer understandable to a human being, and hence can not be realistictly expected to be able to modify it. To make matters worse, translation is, for the most part, one way only, so it cannot be "untranslated" back into a high level language. Only the owner of the original program before it was compiled (the "source code") can change it and then re-compile it again.
The issue is, of course, more complex than that, but that's the short explanation.
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