What you describe is often called a 'chat bot' today. People have been working on these since the 1960s; the most famous being ELIZA.
I would forget about any notions about writing a program which can 'understand English like a human'. While I may believe that computers posses the ability for intelligence, I don't think it will be done by breaking up sentences into component parts.
I have spent many years trying (since around 1986). You can find my last and final embarrasing effort here:
http://www.generation5.org/content/2001/answer.asp
There has been a lot of effort in developing these kind of programs, a field generally called NLP (natural language processing). In the 80s, people used to talk about 'expert systems'. Recently, there has been an effort called CYC - which maybe worth taking a look at, of only to see what you are getting into.
Personally, I would either quit or keep your expectations very low.
