design error

This morning, I meet Mr.Tliu at 615. After I started my computer, he told me there were a lot of design error in my program for the dictionary. And I started my program, he analysised some errors in the program, and gave me many constructive advices.
After the examing of the program I use a famous dictionary's program, and I found some other errors in my program.
Why there were so many errors? I think there were two reasons.
Firstly, when I was programing for it, I wanted to complete a primary version, and then to keep consummating it. And this is a good working model for me right now, I think.
Secondly, although I have completed a lots of small projets before I joined in IR, I have a lot of flaws in programing. I should improve it. Right now, we are studying Soft Engineering, I want to study it well firstly.