i can understand english well but not speak well,why?

Speak a little English every day. The absolute best way to learn any new language is just to speak it. It doesn't matter if you only know five English words or if you're practically fluent -- speaking English with another person is the fastest, most effective method of improving. Don't wait until you "feel more comfortable" speaking in English -- you probably won't reach that level for a long time, so push yourself outside of your comfort zone and start speaking English today.

I know

practice every day not much but a little talk every place you find where you can speak it....or practice it in front of a mirror