AnewcomerWhy do you keep quiet when I ask
you?
This is difficult to respond to because it's difficult to
understand what you are trying to say. Are you trying to say what
a teacher says to a reluctant student? If so, say:
Why do you keep quiet when I call on you?
Why do you keep quiet whenever I call on you?
Why do you keep quiet every time I call on you?
You can use when or whenever or every time, but not more than one of them at a time.
You can substitute freeze up for keep quiet in any of the three sentences above if you want to indicate inability to speak because of fear.
_________
If the example is not that of a teacher, then try one of these:
Why don't you answer my question?
Why don't you answer when I ask you a question?
CJ