Hi Pearl Jiang,

Just say the same thing you said in your question. "I'm sorry my schedule is full and I have several language partners already". Tell the truth with a smile and people will understand. Hope this helps ! :)

Since I was afraid that they will be unhappy. But finally I still said the truth after receiving these comments. :-)

Thank you. ^_^

   Pearl:

 I respond to  requests all requests,  whether it is to politely decline to help  or to propose that

someone  practice English with me.  I do accept   language partners  when possible, providing that the hours  that I teach  are convenient  for the language partner.

You can just say: Sorry i don't have time. see you later.

Or explain that in your status!

Gobbit:

Hmmmmmmmmmm, actually I don't know.XD

Maybe, but I'm not sure. One reason why I asked this question is that I wasn't a confident person  before. I received my friends' encouragement so that I have improved now. But sometimes I still worried about how to refuse people when I cannot or I don't like to do so. :-)